Mythology
The mythology of pre-Christian Ireland did not entirely survive the conversion to Christianity, but much of it was preserved, shorn of its religious meanings, in medieval Irish literature, which represents the most extensive & best preserved of all the branches of Celtic mythology.
Although many of the manuscripts have failed to survive, & much more material was probably never committed to writing, there is enough remaining to enable the identification of distinct, if overlapping, cycles: the Mythological Cycle, the Ulster Cycle, the Fenian Cycle & the Historical Cycle. There are also a number of extant mythological texts that do not fit into any of the cycles. Additionally, there are a large number of recorded folk tales that, while not strictly mythological, feature personages from one or more of these four cycles.