Brigid of Kildare in the context of "Brigid's cross"

Christianity has been the largest religion in Ireland since the 5th century. After a pagan past of Antiquity, missionaries (most famously including Saint Patrick) converted the Irish tribes to Christianity in quick order. This produced a great number of saints in the Early Middle Ages, as well as a faith interwoven with Irish identity for centuries since − though less so in recent times.

Most Christian churches are organized on an "all-Ireland" basis, including both the Republic of Ireland and Northern Ireland. In the 2022 census, 76.1% of residents in the Republic of Ireland identified as Christians: 69.1% as Catholics, 4.2% as Protestants, 2.1% as Orthodox Christians and 0.7% as other Christians. In the 2021 Northern Irish census, 79.7% of residents identified as Christians: 42.3% as Catholics, 16.6% as Prebysterian, 11.5% as Church of Ireland members, 2.4% as Methodist and 6.9% as other Christians. Orthodoxy has been the fastest growing branch of Christianity in Ireland since 1991.

Saint Patrick (Latin: Pātricius; Irish: Pádraig Irish pronunciation: [ˈpˠɑːɾˠɪɟ] or Irish pronunciation: [ˈpˠaːd̪ˠɾˠəɟ]; Welsh: Padrig) was a fifth-century Romano-British Christian missionary and bishop in Ireland. Known as the "Apostle of Ireland", he is the primary patron saint of Ireland, the other patron saints being Brigid of Kildare and Columba. He is also the patron saint of Nigeria. Patrick was never formally canonised by the Catholic Church, having lived before the current laws were established for such matters. He is venerated as a saint in the Catholic Church, the Lutheran Church, the Church of Ireland (part of the Anglican Communion), and in the Eastern Orthodox Church, where he is regarded as equal-to-the-apostles and Enlightener of Ireland.

The dates of Patrick's life cannot be fixed with certainty, but there is general agreement that he was active as a missionary in Ireland during the fifth century. A recent biography on Patrick shows a late fourth-century date for the saint is possible. According to tradition dating from the early Middle Ages, Patrick was the first bishop of Armagh and Primate of Ireland, and is credited with bringing Christianity to Ireland (despite evidence of some earlier Christian presence on the island), and converting Ireland from paganism in the process.

Imbolc or Imbolg (Irish pronunciation: [ə ˈmˠɔlˠəɡ]), also called Saint Brigid's Day (Irish: Lá Fhéile Bríde; Scottish Gaelic: Là Fhèill Brìghde; Manx: Laa'l Breeshey), is a Gaelic traditional festival on 1 February. It marks the beginning of spring, and in Christianity, it is the feast day of Saint Brigid, Ireland's patroness saint. Historically, its many folk traditions were widely observed throughout Ireland, Scotland and the Isle of Man. Imbolc falls about halfway between the winter solstice and the spring equinox and is one of the four Gaelic seasonal festivals, along with Bealtaine, Lughnasadh and Samhain.

Imbolc is mentioned in early Irish literature, although less often than the other seasonal festivals. Historians suggest that Imbolc was originally a pre-Christian (or pagan) festival associated with the lambing season, the coming of spring, and possibly the goddess Brigid, proposing that the saint and her feast day might be Christianizations. A feast of Saint Brigid was first mentioned in the Middle Ages, but its customs were not recorded in detail until the early modern era. In recent centuries, Brigid's crosses have been woven on St Brigid's Day and hung over doors and windows to protect against fire, illness, and evil. People also made a doll of Brigid (a Brídeóg), which was paraded around the community by girls, sometimes accompanied by 'strawboys'. Brigid was said to visit one's home on St Brigid's Eve. To receive her blessings, people would make a bed for Brigid, leave her food and drink, and set items of clothing outside for her to bless. Holy wells would be visited, a special meal would be had, and the day was traditionally linked with weather lore.

Dar Lugdach (also Darlugdach died c. 525/527) was the immediate successor of Brigid of Kildare as abbess of Kildare, and is recognised as a saint. She is recorded as having died one year to the day after Brigid, and shares the same feast day as the more famous abbess. Little is known of her family history.