The Partholonians
Partholonians were said to have landed in Ireland at Beltaine, where they lived for three hundred years. According to myth, they battled with the Fomhóire, a race of mis-shapen beings, probably representing the aboriginal gods of the land as there is no mention of when the Fomhóire arrived. The whole race of the Partholonians were mysteriously wiped out by a plague.
Source: http://www.mediatel.lu/team/nmack/mmp_myth_partholonians.html