our saga begins in ireland and britain in the 7th century ad [ ad is the abbreviation of medieval latin anno domini translated as in the year of our lord used to label or number years in the christian era - also known as the `` common era '' ] britain : the roman legions which had occupied much of britain for almost 400 years , but not the northern part , which was inhabited by the caledonians or picts , withdrew in ad 409 .