>>For all I know, "20 and 10" is a fairly typical way of saying "30" in Irish.<<
The same work contains the usual form of thirty -'Tricha' where he is described as being 30 feet tall.
>>"Archaic" words alone are no indication of the age of a composition. The
poets learned archaic words as part of their professional education and
were well able to insert them wherever they needed them. Only old - and
correct - verbal forms can serve as good indicators of old age.<<