Who is Saint Oliver Plunkett ?

On September 30, 1629 Oliver Plunkett was born at his family home at Loughenew, in the county of Meath. He was the last Catholic to die for his faith at Tyburn and the first of the Irish martyrs to be beautified. Oliver Plunkett was educated by the Jesuits at the then newly established Irish College, He was ordained in 1654. Oliver Plunkett was appointed Archbishop of Armagh and Primate of all Ireland by Pope Clement IX.
He established the Jesuits in Drogheda, where they ran a school for boys, and a college for theologian students, he even extended his ministry to Gaelic speaking Catholics of the highlands and the isles of Scotland.
In 1679 Oliver Plunkett was arrested and put on trial at Dundalk, for conspiring against the state by plotting to bring 20,000 French soldiers into the country and leveling a tax on his clergy to support 70,000 men for rebellion. Lord Shaftesbury under King Charles II knew that the Archbishop would never be convicted in Ireland, he was therefore removed to Newgate prison in London.
At the first trial, the grand jury found no true bill: he was not released, but it was adjourned till June 1681. There is doubt that the court had jurisdiction over the Irishman, and the second trial was conducted with only a semblance of justice, so that Lord Campbell, writing of the judge, Sir Francis Pemberton, calls it a disgrace to himself and his country.
Archbishop Plunkett was found guilty by the jury for high treason, and the Primate of All Ireland was condemned to be hanged, disemboweled. and quartered. The execution took place July 1 1681. There were several priests present at the gallows, and one of them, Father Gasper, a Belgian Carmelite attached to the Spanish Embassy, pronounced the words of absolution from sin as the still-living body was cut down. The hangman removed the head from the body and threw it into the fire, from which James McKenna rescued it before the fire had time to do more than minimal damage. Father Travers, an English Carmelite, wrote joyfully, "I had the honor of holding the sacred head and the quarters of venerable martyr in my own hands and placing them in the tin chest after the cruel sentence had been carried out in all its gruesome details".
The body was placed in two tin boxes and buried next to five Jesuits who had gone before him. When Father Corker was released from Newgate prison two years later he had the remains moved to Benedictine Monastery in Lambspring Germany, two hundred years later in 1883 the body was transported to Downside Abbey, England where it is enshrined today. The martyr's head is preserved in St. Peters Church at Drogheda. In the Summer of 1975 some of the remains were returned to Ireland. Oliver Plunkett was beautified in 1920 and canonized in 1975.

More information on St. Oliver Plunkett can be found in a book:

A Martyr Bishop, The life of St. Oliver Plunkett: by John McKee