Mad King Sweeney is a character in Flann O'Brien's At Swim-Two-Birds, so I read Seamus Heaney's translation of the mediæval Irish book about him, Sweeney Astray.

The old Irish story itself reuses ideas from Apuleius's The Golden Ass and probably Ovid's Metamorphoses. I haven't read Ovid yet but as it's also the inspiration for Christoph Ransmayr's The Last World I have another reason to get it off the pile.