Sweeney is very much a literary creation rather than a creature of folktale, though this discussion is faily wide ranging. In terms of psychosis as a breakdown of nature/nurture duality then Sweeney is hard to beat. Proto Gnostic horror? In any case as a literary device to explore the ambiguities of nature in verse it's a piece of genius - and very modern. Whilst the Heaney translation is beautiful, you'll find Suibhne at his most profound amongst the pages of Flann O' Brien's stunning debut At Swim Two Birds where he inspires the following:

When stags appear on the mountain high
with flanks the colour of bran,
when a badger bold can say good-bue,
A PINT OF PLAIN IS YOUR ONLY MAN