Plunkett is born on November 1, 1625, in Loughcrew, County Meath, to well-to-do parents with Hiberno-Norman ancestors. 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. In the aftermath, the public practice of Roman Catholicism is banned and Roman Catholic clergy are executed. As a result, it is impossible for Plunkett to return to Ireland for many years.
He eventually sets foot on Irish soil again on March 7, 1670, as the English Restoration of 1660 has begun on a basis of toleration. After arriving back in Ireland, he sets about reorganising the ravaged Roman Church and builds schools both for the young and for clergy, whom he finds “ignorant in moral theology and controversies.” The Penal Laws have been relaxed in line with the Declaration of Breda in 1660 and Plunkett is able to establish a Jesuit College in Drogheda in 1670.
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 refusing 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, Plunkett refuses to leave his flock. At some point before his final incarceration, he takes refuge in a church that once stood in the townland of Killartry, in the parish of Clogherhead in County Louth, seven miles outside Drogheda. He is arrested in Dublin in December 1679 and imprisoned in Dublin Castle. Plunkett 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.
Plunkett is found guilty of high treason in June 1681 “for promoting the Roman faith” and is condemned to death. He is hanged, drawn, and quartered at Tyburn on July 1, 1681, the last Roman Catholic martyr to die in England.
Oliver Plunkett is beatified on May 23, 1920 and canonised in 1975, the first new Irish saint for almost seven hundred years, and the first of the Irish martyrs to be beatified. For the canonisation, the customary second miracle is waived. Plunkett has since been followed by 17 other Irish martyrs who were beatified by Pope John Paul II in 1992. Among them are Archbishop Dermot O’Hurley, Margaret Ball, and the Wexford Martyrs.