What religion was Ireland before Christianity?

Answer 1

Some variant of what is now considered a general Celtic mythology that also was in Whales, Scotland and pre-roman Britain. This included a pantheon of various nature deities.

Ireland in particular believed in the existence of the Tuatha De Dannan, a spirit-like group of beings who inhabited the island before the later Celtic migration. After Christianity the former deities and spirit beings became the fairies of modern folk traditions.

Answer 2

The Irish people followed a religion known as Druidism, additionally they had their own form of polytheism following the Tuatha de Dannen, or the people of Danu. This branch of religion had many similarities with the better known Norse Pantheon and Greek Pantheon. However, one major exception was that Celtic Peoples, (and their religion) tended to be matriarchal in structure rather than patriarchal. For example, the main Deity of War was the Trippled-Aspect Morrigu or Morrigan; all three aspects of the Morrigu were female.