A number of law codes have in the past been in use in the various Celtic nations since the Middle Ages.
While these vary considerably in details, there are certain points of similarity.
The Brehon Laws governed everyday life and politics in Ireland until the Norman invasion of 1171 (the word "Brehon" is an Anglicisation of "breitheamh" (earlier "brithem"), the Irish word for a judge). The laws were written in the Old Irish period (ca. 600–900 AD) and probably reflect the traditional laws of pre-Christian Ireland.
The codification of Welsh law has been traditionally ascribed to Hywel Dda, king of most of Wales between 942 and his death in 950. This was partly an adaptation of previously existing laws however. Welsh law remained in force in Wales until the death of Llywelyn ap Gruffudd in 1282 for criminal cases, and until the Laws in Wales Acts in the mid-sixteenth century for civil cases.
Common features of these codes include an emphasis on the payment of compensation for a crime to the victim or the victim's kin rather than on punishment by the ruler. In other words, all law was tort law, with no "victimless" crimes or crimes against the State.
Celtic laws in late prehistory.
While occasional references to "common Celtic law" in academic literature, such as Fergus Kelly's "Guide to Early Irish Law", seem to imply that there was one original Celtic law from which the various later Celtic laws, some of which are historically attested (see Brehon law, Cyfraith Hywel), evolved, it is unlikely that anything like 'original Celtic law' (or 'common Celtic law') ever existed as a unified, let alone a codified body of law. Rather, it is currently thought that various central and western European societies in later prehistory, commonly lumped together under the name 'Celts', had individually different customary laws, which evolved out of similar social needs, influenced each other considerably over several centuries or even millennia, and thus ended up reasonably similar to each other.
'Original (or Common) Celtic law' thus can only be reconstructed, and only as a generalisation. Such a generalisation does not reflect actual past legal practice, but can only show which general principles are likely to have been typical for many (but not necessarily all) early Celtic laws.
Basic workings.
Celtic law evolved from the judgements of private competing judges. Murray Rothbard describes the system this way:
The basic political unit of ancient Ireland was the tuath. All “freemen” who owned land, all professionals, and all craftsmen, were entitled to become members of a tuath. Each tuath’s members formed an annual assembly which decided all common policies, declared war or peace on other tuatha, and elected or deposed their “kings.” An important point is that, in contrast to primitive tribes, no one was stuck or bound to a given tuath, either because of kingship or of geographical location. Individual members were free to, and often did, secede from a tuath and join a competing tuath.
Celtic law is thus a stateless form of law like most customary law forms. "... the professional jurists were consulted by parties to disputes for advice as to what the law was in particular cases, and these same men often acted as arbitrators between suitors. They remained at all times private persons, not public officials; their functioning depended upon their knowledge of the law and the integrity of their judicial reputations."
After the private judge, chosen by the disputants, has made his decision, how was the judgement – the compensation to the victim – enforced?
Through an elaborate, voluntarily developed system of “insurance,” or sureties. Men were linked together by a variety of surety relationships by which they guaranteed one another for the righting of wrongs, and for the enforcement of justice and the decisions of the brehons. In short, the brehons themselves were not involved in the enforcement of decisions, which rested again with private individuals linked through sureties.
Dating and definition.
It is problematic to date ‘Common Celtic law’, and the best we can arrive at are rough estimates. It is quite apparent, by parallels existing between Celtic and other Indo-European laws that at least some of the legal principles that make up Common Celtic law must be very ancient, perhaps going back to the Early Bronze Age or even the Neolithic period. Others may have only developed much later, perhaps even only as a result of contacts with Mediterranean cultures (mainly Greeks and Romans) during the late Iron Age and Antiquity.
In some cases, cognate terms used for parallel practices in the early medieval Irish and Welsh laws allow us to assume that these practices were already used in what is referred to by linguists as the 'common Celtic period', which is usually dated to around 1000 BC (e.g. Kelly 1988, 231), although this at best is a very rough estimate. Where parallels for such practices exist, but with non-cognate terminology, in other Indo-European laws, we can start to consider the cognate practices as specifically Celtic forms of law. Similarly, where such cognate terminology exists for parallel practices in the early medieval Celtic laws, but not in other Indo-European laws, we can consider these to be specifically Celtic laws.
As there are hardly any characteristics of Celtic law that cannot be found in at least some other, non-Celtic laws as well, the most sensible definition of Celtic law seems to be one that focuses on the language in which the law is (usually) dispensed. As such, 'Celtic law' would be any law (usually) dispensed in a Celtic language.
Sources.
Quite generally, the oldest textual sources for Celtic laws which give us at least a very general idea of actual practice date from the late Iron Age, roughly the last two centuries BC and the first century AD.
Some evidence can be gathered from the 'usual suspects', like Caesar's "De Bello Gallico", who discusses some aspects of Celtic laws in his account of the Gaulish Wars, specifically his famous excursus on the Gauls, but also in some other passages. Some of these passages allow us to confirm the existence, in some Iron Age Celtic laws, of some of the legal principles which can be reconstructed from early medieval Celtic laws as likely elements of common Celtic law, increasing the likelihood of any such generalised reconstruction. Other passages can tell us about particular legal practices in individual Gaulish societies, which are specific for just the society described, without allowing any greater generalisations.
Besides some references in classical authors, there is a small number of texts in Iron Age Celtic languages, some of which (may) contain legal information, too. The most clearly legalistic sources are the Celtiberian inscriptions on Bronze tablets from Contrebia Belaisca (Botorrita), dating from early after the Roman occupation of this area. Botorrita IV might even start with a legalistic formula, '[ tam : tirikantam : entorkue : toutam [|] : sua kombal[ke]z : ...' which could perhaps be interpreted as '...the senate and the people have decided...', mirroring the Roman equivalent. However, these texts are notoriously hard to interpret and not very long either. As such, they are of only limited value, at best allowing to speculate about local legal customs.
To some degree, exceptionally short pieces of textual evidence in Celtiberian also allow to gain some information about what possibly could have been a widespread Celtic legal practice. From Spain, a number of so-called tesserae hospitales, 'hospitality tablets', are known, inscribed in Celtiberian, often with no more than a single word, occasionally with very short sentences. These may indicate a custom of granting hospitality to foreigners, which may have had a basis in customary law and may have allowed to grant legal protection to foreigners, as also found in many other Indo-European societies and the early Irish and Welsh laws.
Finally, there are the archaeological sources, which abound, but are almost impossible to interpret as to their possible legal meanings. Of course, it may occasionally be possible to speculate that an archaeological feature, say, the remains of a fence, may have expressed some legal concept, e.g. ownership of property. But other than that, archaeology remains mostly silent. At best, archaeological evidence can help to strengthen an argument based on reconstructive generalisations from early medieval Irish and Welsh laws, ideally such that are also supported by evidence from historical texts.
Principles of Celtic laws in late Prehistory.
A number of such legal principles, which most likely were widespread in early Celtic laws, can be reconstructed with reasonable degrees of probability. They are mostly centred around kinship and contractual relations, although we have some ideas about criminal law and legal procedure as well. For all of these, we also find reasonably similar principles in either Roman and/or Germanic laws, and in most cases also in other Indo-European laws, making it quite likely that these reconstructions are roughly accurate, even if they lack in detail. Given that many, if not most of them come with an internal Celtic cognate terminology, it is unlikely that they actually are late loans from e.g. Roman provincial law, although some crossovers in legal customs should be assumed. In at least some cases, e.g. in contract law, a co-evolution of Roman, Germanic and Celtic legal systems, based on intensive contact, is likely, even though the contract laws of each subgroup of these larger collectives may already have started out reasonably similar.
Kinship.
Kinship without a doubt played a very important role in Celtic societies in late prehistory. The importance that ancestry had for the late prehistoric Celts is stressed by several classical authors, and seems – at least for some areas, in some periods – also be confirmed in the archaeology by the effort put into burials. The structure of Celtic kin-groups can be reconstructed to some extent, but little of internal kinship relations will have been formalised in a way that could be considered law. There are, however, a number of important legal principles that can be reconstructed, which are related to kinship or external kin-group relations. There is relatively strong evidence for a customary requirement for kin members to support and help each other, in everyday life as much as in legal disputes. This seems to be evident from historical sources, and would fit well with what we find in the early medieval Irish and Welsh laws.
One of the most important legal principles that seems to have been associated with kinship is that of private property, especially the ownership of land and resources. In the very least, differential access to property and resources for different groups in society is evident in the archaeology, indicated both by differential burial wealth and relatively consistent enclosing of settlement space. Most likely, access rights were at least partially based on kinship/descent, as this is the case not only in the early medieval Irish and Welsh laws, but also in the neighbouring Roman and Germanic laws. Even though we cannot be perfectly sure, inheritable individual possession of property and resources, with legal ownership resting with the wider kin-group, is the most likely form of regulating differential access to property and resources in Celtic societies in late prehistory.
The other highly significant legal aspect associated with kinship relations is of sexual unions and reproduction. Inheritance seems to have been passed on primarily in the paternal line, as such, clarifying the relations between partners, who probably quite frequently were members of different kin-groups, as well as their children, must have been quite essential. The surprisingly close parallels between the early medieval Irish and Welsh laws where sexual unions are concerned, and the similarity between the most prestigious union described in them with the Gaulish marriage as described by Caesar, indicate that the treatment of different sexual unions was quite similar over wide areas of western Europe from late prehistory well into the medieval period.
We also know that, at least amongst the Gaulish and the early medieval Irish nobility, polygyny was a widespread practice. Given the detail given to different kinds of sexual union in early medieval Welsh law, it seems reasonable to assume that polygyny was also common in Wales some time before the law-texts were put into writing. Given that the main focus in the early medieval Irish laws where sexual unions are concerned are with the contribution to and division of assets of the union in case of divorce, as well as the responsibilities towards children resulting from these unions, it is quite likely that similar provisions