One of the reasons that potato cultivation was so widespread in
Ireland was that it could be grown virtually anywhere under the
"lazybed" technique, versions of which were employed by the
Incas and are widely in use today. Just place the tubers on
peat, manure or whatever, dig a drainage trench on either side
while covering the tubers, and wait until harvest. Grains could
not have been cultivated in most of the places where potatos
thrived, and the peasants who grew most of the potatos for
subsistence did not have access to the more arable land. For a
vivid account of this, see Henry Hobhouse, _Seeds of Change:
Five Plants that Transformed Mankind_ (NY: Harper & Row, 1986).
I'm sure there are better accounts. On the question of genetic
diversity (raised in another posting), the genetic stock for
Irish potatoes apparently originated in the tropical Andes, and
was introduced to Europe ca. 1580. Early potatos produced poorly
in Europe because they were adapted to a more tropical
photoperiod. Before they were sui