William Tirry (Irish: Liam Tuiridh) OSA (1609 – 12 May 1654) was an Irish Roman Catholic priest of the Order of Saint Augustine following the Cromwellian conquest of Ireland. He was captured by the priest hunters at Fethard, County Tipperary while continuing his priestly ministry covertly and was hanged at Clonmel, officially for high treason against the Commonwealth of England, but in reality as part of The Protectorate's systematic religious persecution of the Catholic Church in Ireland. Pope John Paul II beatified Friar William Tirry as one of the 24 officially recognized Irish Catholic Martyrs in 1992.

Tirry was born into a well-to-do family of Hiberno-Norman merchants in Cork, Ireland in 1608, the son of Robert and Joan Tirry. He was named after his uncle, the elder William Tirry, Bishop of Cork and Cloyne. Tirry was the grandson of Edmond Terry, or Tirry, Lord Mayor of Cork, and his wife Catherine Galway. His aunt Joan married Dominick Sarsfield, 1st Viscount Sarsfield, the Chief Justice of the Irish Common Pleas: their son William, the 2nd Viscount, played an important role in Tirry's life as his patron and protector.

Well-educated, he learned Ecclesiastical Latin and Koine Greek, but also spoke the Classical Gaelic literary language and the Munster Irish vernacular. At the age of eighteen, he left to study for the priesthood in Catholic Europe. He is known to have studied for the priesthood first at the Irish College in Valladolid and then at the Collège des Grand Augustins in Paris, and that he was ordained around 1634. He and then spent five years (1636–1641) in Brussels, Belgium.

After his return to Ireland, he joined the Augustinian Order at St. Austin's Abbey in Cork City. He then spent about four months working as secretary to his uncle, Bishop William Tirry. Although he relinquished this post to return to St Austin's Abbey, he was then persuaded to act as chaplain to his cousin Lord Kilmallock and as tutor to the latter's son and heir.

Tirry was elected Provincial Secretary in 1646. Lord Sarsfield of Kilmallock's death in 1648 deprived Tirry of his chief protector. In 1649, he was chosen as prior of the Augustinian convent in Skreen, County Mayo, but was unable to assume his duties there because of the beginning of the Cromwellian conquest of Ireland. A law was enacted on 6 January 1653 declaring that any Roman Catholic priest in Ireland was guilty of high treason. Tirry was accordingly forced into hiding like all other priests.

All sources are agreed that for three years prior to his capture, Friar William Tirry found shelter with his distant cousin, a local Old English noblewoman and elderly widow named Mrs. Amy Everard, at Fethard, County Tipperary. Aside from acting as Mrs. Everard's son's tutor and continuing his covert priestly ministry, "to all who came seeking the sacraments", Friar William spent most of his time in secret prayer and acts of penance.

Fr. William Tirry was arrested at Fethard while vested for Mass on Holy Saturday, 25 March, 1654. Both priestly vestments and samizdat writings in defense of the Catholic Faith were also confiscated during his arrest. To the fury of the judges, one of the discoveries at Tirry's arrest was, "a manuscript work composed by him discrediting Protestantism". Fr. Tirry was taken to Clonmel Gaol and held there pending trial. Three men had reported his whereabouts to the priest hunters in return for the £5 bounty.

According to the Tipperary Museum of Hidden History, the earliest records of a Clonmel city gaol date from 1650 and refer to a small building located around what is now O'Connell Street. According to the Museum, "Prisons were run by private individuals. Gaolers had no concern for the physical or moral well being of their prisoners. Those who could afford it, could pay the gaoler in order to buy themselves some comforts while imprisoned such as private rooms, family visits, food and even drink. Overcrowding, disease and escapes were common."