Oliver Plunkett was born on the 1st of November 1625 (earlier biographers gave his date of birth as (1 November) 1629, but 1625 has been the consensus since the 1930s) in Loughcrew, County Meath, Ireland to well-to-do parents with Hiberno-Norman ancestors. He was related by birth to a number of landed families, such as the recently ennobled Earls of Roscommon, as well as the long-established Earls of Fingall, Lords Louth and Lords Dunsany. Until his sixteenth year, the boy’s education was entrusted to his cousin Patrick Plunkett, Abbot of St Mary’s, Dublin, and brother of Luke Plunkett, the first Earl of Fingall who later became Bishop, successively, of Ardagh and of Meath. As an aspirant to the priesthood, he set out for Rome in 1647, under the care of Father Pierfrancesco Scarampi, of the Roman Oratory. At this time, the Irish Confederate Wars were raging in Ireland; these were essentially conflicts between native Irish Roman Catholics, English, and Irish Anglicans and Protestants. Scarampi was the Papal envoy to the Roman Catholic movement known as the Confederation of Ireland. Many of Plunkett’s relatives were involved in this organisation.
He was admitted to the Irish College in Rome and he proved to be an able pupil. He was ordained a priest in 1654, and deputed by the Irish bishops to act as their representative in Rome. Meanwhile, the Cromwellian conquest of Ireland (1649–53) had defeated the Roman Catholic cause in Ireland and, in the aftermath, the public practice of Roman Catholicism was banned and Roman Catholic clergy were executed. As a result, it was impossible for Plunkett to return to Ireland for many years. He petitioned to remain in Rome and, in 1657, became a professor of theology. Throughout the period of the Commonwealth and the first years of Charles II‘s reign, he successfully pleaded the cause of the Irish Roman Church, and also served as theological professor at the College of Propaganda Fide. At the Congregation of Propaganda Fide on 9 July 1669, he was appointed Archbishop of Armagh, the Irish primatial see, and was consecrated on 30 November at Ghent by the Bishop of Ghent, Eugeen-Albert, count d’Allamont. He eventually set foot on Irish soil again on 7 March 1670, as the English Restoration of 1660 had started on a tolerant basis. The pallium was granted him in the Consistory of 28 July 1670.
After arriving back in Ireland, he set about reorganising the ravaged Roman Church and built schools both for the young and for clergy, whom he found “ignorant in moral theology and controversies”. He tackled drunkenness among the clergy, writing: “Let us remove this defect from an Irish priest, and he will be a saint”. The Penal Laws had been relaxed in line with the Declaration of Breda in 1660 and he was able to establish a Jesuit College in Drogheda in 1670. A year later 150 students attended the college, no fewer than 40 of whom were Protestant, making this college the first integrated school in Ireland. His ministry was a successful one and he is said to have confirmed 48,000 Catholics over a 4-year period. The government in Dublin, especially under the Lord Lieutenant of Ireland, the Duke of Ormonde (the Protestant son of Catholic parents) extended a generous measure of toleration to the Catholic hierarchy until the mid-1670s