Also known as the Fool of the Forth, Amadan-na-Briona is one of the most dangerous inhabitants of faery land, whose touch causes an incurable illness, leading to certain death. Not even Biddy Early, the greatest of the fairy doctors, could cure his stroke.
Amadan-na-Briona is most dangerous on Midsummer's eve in June, for that is when he is out and abroad. He also carries away folks, and it is said that "to meet the Amadan is to be in prison forever" (Lady Gregory, p. 254). The Fool of the Forth also seems to be a shape-changer, able to appear an innocent child or a horrible beast.
Tales of Amadan seem to be limited to Galway.
Sources
Visions and Beliefs in the West of Ireland by Lady Gregory.