Saint Patrick in the context of "St. Patrick's Day"

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 Brigid of Kildare or Saint Brigid of Ireland (Irish: Naomh Bríd; Classical Irish: Brighid; Latin: Brigida; c. 451 – c. 525) is the patroness saint (or 'mother saint') of Ireland, and one of its three national saints along with Patrick and Columba. According to medieval Irish hagiographies, she was an abbess who founded the important abbey of Kildare (Cill Dara), as well as several other convents of nuns. There are few documented historical facts about her, and her hagiographies are mainly anecdotes and miracle tales, some of which are rooted in pagan folklore. They say Brigid was the daughter of an Irish clan chief and an enslaved Christian woman, and was fostered in a druid's household before becoming a consecrated virgin. She is patroness of many things, including poetry, learning, healing, protection, blacksmithing, livestock, and dairy production. In her honour, a perpetual fire was kept burning at Kildare for centuries.

Some historians suggest that Brigid is a Christianisation of the Celtic goddess Brigid. The saint's feast day is 1 February, and traditionally it involves weaving Brigid's crosses and many other folk customs. It was originally a pre-Christian festival called Imbolc, marking the beginning of spring. Since 2023, it has been a public holiday in the Republic of Ireland. This feast day is shared by Dar Lugdach, who tradition says was her student, close companion, and successor.

The Catholic Church in Ireland, or Irish Catholic Church, is part of the worldwide Catholic Church in communion with the Holy See. With approximately 4.3 million members, it is the largest Christian church in Ireland. In the Republic of Ireland's 2022 census, 69% of the population identified as Roman Catholic, and in Northern Ireland's 2021 census, 42.3% identified as Roman Catholic.

The Archbishop of Armagh, as the Primate of All Ireland, has ceremonial precedence in the church. The church is administered on an all-Ireland basis. The Irish Catholic Bishops' Conference is a consultative body for ordinaries in Ireland.Christianity has existed in Ireland since the 5th century and arrived from Roman Britain (most famously associated with Saint Patrick), forming what is today known as Gaelic Christianity. It gradually gained ground and replaced the old pagan traditions. The Catholic Church in Ireland cites its origin to this period and considers Palladius as the first bishop sent to the Gaels by Pope Celestine I. However, during the 12th century a stricter uniformity in the Western Church was enforced, with the diocesan structure introduced with the Synod of Ráth Breasail in 1111 and culminating with the Gregorian Reform which coincided with the Anglo-Norman invasion of Ireland.

Downpatrick (from Irish Dún Pádraig, meaning 'Patrick's fort') is a town in County Down, Northern Ireland. It is on the Lecale peninsula, about 21 mi (34 km) south of Belfast. In the Middle Ages, it was the capital of the Dál Fiatach, the main ruling dynasty of Ulaid. Its cathedral is said to be the burial place of Saint Patrick. Today, it is the county town of Down and the joint headquarters of Newry, Mourne and Down District Council.Downpatrick had a population of 11,545 according to the 2021 Census.

Armagh (/ɑːrˈmɑː/ ar-MAH; Irish: Ard Mhacha, IPA: [ˌaːɾˠd̪ˠ ˈwaxə], "Macha's height") is a city and the county town of County Armagh, in Northern Ireland, as well as a civil parish. It is the ecclesiastical capital of Ireland – the seat of the Archbishops of Armagh, the Primates of All Ireland for both the Roman Catholic Church and the Church of Ireland. In ancient times, nearby Navan Fort (Eamhain Mhacha) was a pagan ceremonial site and one of the great royal capitals of Gaelic Ireland. Today, Armagh is home to two cathedrals (both named after Saint Patrick) and the Armagh Observatory, and is known for its Georgian architecture.

Statistically classed as a medium-sized town by NISRA, Armagh was given city status in 1994 and Lord Mayoralty status in 2012. It had a population of 16,310 people in the 2021 Census.

The Primacy of Ireland belongs to the diocesan bishops of the Irish dioceses with highest precedence. The Archbishop of Armagh is titled Primate of All Ireland and the Archbishop of Dublin Primate of Ireland, signifying that they are the senior clerics on the island of Ireland, the Primate of All Ireland being the more senior. The titles are used by both the Catholic Church in Ireland and Church of Ireland.

Primate is a title of honour, and in the Middle Ages there was an intense rivalry between Armagh and Dublin as to seniority. The Archbishop of Armagh's leading status was based on the belief that his see was founded by St. Patrick, making Armagh the ecclesiastical capital of Ireland. On the other hand, Dublin, after the Anglo-Norman invasion of Ireland, was the administrative centre of the country, and its largest city. The dispute between the two archbishoprics was settled by Pope Innocent VI in 1353, with occasional brief controversy since. The distinction mirrors that in the Church of England between the Primate of All England, the Archbishop of Canterbury, and the Primate of England, the Archbishop of York.

Saint Patrick's Saltire or Saint Patrick's Cross is a red saltire (X-shaped cross) on a white field. In heraldic language, it may be blazoned argent, a saltire gules. Saint Patrick's Flag (Irish: Bratach Naomh Pádraig) is a flag composed of Saint Patrick's Saltire. The origin of the saltire is disputed. Its association with Saint Patrick dates from the 1780s, when the Anglo-Irish Order of Saint Patrick adopted it as an emblem. This was a British chivalric order established in 1783 by George III. It has been suggested that it derives from the arms of the powerful Geraldine or FitzGerald dynasty. Some Irish nationalists opposed the usage of the flag to represent Ireland, as they considered it to be a British invention.

After its adoption by the Order of Saint Patrick, it began to be used by other institutions. When the Acts of Union 1800 joined the Kingdom of Ireland with the Kingdom of Great Britain, the saltire was added to the British flag to form the Union Flag still used by the United Kingdom. The saltire has occasionally served unofficially to represent Northern Ireland and also appears in some royal events.