AS in England, the Tudor Reformation was an act of state in Ireland, implemented by parliamentary legislation. The Reformation was accepted by most of the bishops in 1536 when Papal supremacy was replaced by the supremacy of the state. However, the bishops made no changes in doctrine, and many of the first Reforming bishops are counted in the diocesan lists of both the Catholic Church and the Church of Ireland.

The names of the early Reformers show they were drawn from the mainstream of Irish life - names such as Browne, Butler, Cullen, Devereux, Nugent, Purcell or Walsh - and the episcopal succession continued uninterrupted.

During the reign of Edward VI (1537-1553), a reformed liturgy was introduced from England and the Book of Common Prayer, first used in Christ Church Cathedral, Dublin, on Easter Day, 1551, was the first book printed in Ireland. Under Mary (1553-1558), some Reforming bishops were deposed and married clergy punished, but the Reformation returned under Elizabeth (1558-1603), and was accepted by all but two of the bishops. In 1560, the Irish Parliament again repudiated the authority of the Pope and passed the Act of Uniformity, making Anglicanism the state religion in Ireland.

However, the dividing line between who was Protestant and who was Catholic was not clearly defined even a generation after the Reformation was first introduced. The case of the pluralist Miler Magrath is infamous: he managed to remain Roman Catholic Bishop of Down and Connor while he was the Church of Ireland Archbishop of Cashel.

A more interesting, if less dramatic, example of the confused identities of the day is provided by Magrath's predecessor as Bishop of Achonry. Eugene O'Harte, the Dominican Prior of Sligo, who was appointed Bishop of Achonry on January 28th, 1563, by Pope Pius IV. O'Harte attended the final session of the Council of Trent, but when he returned to the west of Ireland, he was accepted as the bishop of his diocese by old Catholics and new Anglicans alike, and remained in office until he died in his 100th year in 1603.

Historians continue to debate why the Reformation never took hold at a popular level in Ireland. The church historian Canon Michael Burrowes points out in his study of the episcopacy in Ireland: "The reasons why the bulk of the Irish population did not . . . adopt Protestantism, but rather came to look to Rome for alternative structures and pastoral care, remain hotly contested".