Plunkett is born on November 1, 1625 in Loughcrew, County Meath, to parents of Hiberno-Norman ancestors. Until his sixteenth year, his education is entrusted to his cousin Patrick Plunkett, Abbot of St. Mary’s, Dublin, and brother of Luke Plunkett, 1st Earl of Fingall, who later becomes successively Bishop of Ardagh and of Meath. As an aspirant to the priesthood, he sets out for Rome in 1647.
Plunkett is admitted to the Pontifical Irish College in Rome and proves to be an able pupil. He is ordained a priest in 1654, and deputed by the Irish bishops to act as their representative in Rome. Meanwhile, the Cromwellian conquest of Ireland (1649–1653) has defeated the Roman Catholic cause in Ireland. As a result, it is impossible for Plunkett to return to Ireland for many years so he petitions to remain in Rome. At the Congregation of Propaganda Fide on July 9, 1669, he is appointed Archbishop of Armagh and is consecrated on November 30 at Ghent. He returns to Ireland on March 7, 1670, as the English Restoration of 1660 has begun on a basis of toleration.
Plunkett sets about reorganising the ravaged Roman Church and builds schools both for the young and for clergy. The Penal Laws have been relaxed in line with the Declaration of Breda in 1660 and he is able to establish a Jesuit College in Drogheda in 1670, which becomes the first Catholic-Protestant integrated school in Ireland.
On the enactment of the Test Act in 1673, to which Plunkett does not agree for doctrinal reasons, the college is closed and demolished. Plunkett goes into hiding, traveling only in disguise, and refuses a government edict to register at a seaport to await passage into exile.
In 1678 the so-called Popish Plot, concocted in England by clergyman Titus Oates, leads to further anti-Roman Catholic action. Archbishop Peter Talbot of Dublin is arrested, and Plunkett again goes into hiding. Despite being on the run and with a price on his head, he refuses to leave his flock.
Plunkett is arrested in Dublin on December 6, 1679 and imprisoned in Dublin Castle. He is tried at Dundalk for conspiring against the state by allegedly plotting to bring 20,000 French soldiers into the country, and for levying a tax on his clergy to support 70,000 men for rebellion. The trial soon collapses as the prosecution witnesses are themselves wanted men and afraid to appear in court. Plunkett is moved to Newgate Prison in London in order to face trial at Westminster Hall. The first grand jury finds no true bill, but he is not released. The second trial is generally regarded as a serious miscarriage of justice as Plunkett is denied defending counsel.
Archbishop Plunkett is found guilty of high treason in June 1681 “for promoting the Roman faith,” and is condemned to death. Plunkett is hanged, drawn, and quartered at Tyburn on July 1, 1681, the last Roman Catholic martyr to die in England. His body is initially buried in two tin boxes in the courtyard of St. Giles in the Fields church. The remains are exhumed in 1683 and moved to the Benedictine monastery at Lamspringe, near Hildesheim in Germany. His head is brought to Rome, and from there to Armagh, and eventually to Drogheda where it has rested in St. Peter’s Church since June 29, 1921. Most of the body is brought to Downside Abbey, England, where the major part is located today, with some parts remaining at Lamspringe.
Sir Oliver Plunkett is canonised in Rome by Pope Paul VI on October 12, 1975, the first new Irish saint in almost seven hundred years, and the first of the Irish martyrs to be beatified. For the canonisation, the customary second miracle is waived.