Irish pre-history is a pain. Because Celtic culture was non-literary, it wasn’t until the arrival of Christianity that Irish myths and legendary history were written down.
As a result, a lot of the myths were Christianised. St Patrick crops up a bit, meeting legendary figures like Oisín.
One myth that has really suffered is that of the Milesians, the last group to “invade” Ireland and establish Irish culture according to the Lebor Gabála (Book of Conquests). It’s loaded with medieval history and links to Spain, Egypt, the Holy Land and has been discounted as being entirely created by monks.