Ireland remains majority Catholic, though the share identifying as Catholic has fallen substantially from over 90 percent in past decades to roughly 70 percent in the most recent census, with religiously unaffiliated residents now the fastest-growing category at around 12 to 14 percent, alongside small Protestant, Muslim and Orthodox Christian minorities, per Ireland's national census and estimates reflected in Pew Research's Religious Composition of the World, 2010-2020. Christianity reached Ireland substantially through the mission traditionally associated with St Patrick in the fifth century, building on an earlier Gaelic pagan religious culture centered on druids and seasonal festivals such as Samhain and Bealtaine, and Irish monasticism became renowned across early medieval Europe for preserving classical and Christian learning through the so-called Dark Ages, producing illuminated manuscripts such as the Book of Kells.
English and later British rule from the sixteenth century imposed Protestant establishment through the Church of Ireland while the Catholic majority faced severe legal disabilities under the Penal Laws, repealed only gradually from the late eighteenth century through Catholic Emancipation in 1829, a long history of religious discrimination that fed directly into Irish nationalism and the eventual partition of the island in 1921.