He was born into a family of mixed Old English and Gaelic Irish descent, as the son of John Stapleton and his wife Fionnuala (née Nic Conmara).
The book, published in Brussels, was called Catechismus seu doctrina christiana latino-hibernica or, in Irish, Cathcismus sen Adhon, an Teagasc Críostaí iar na foilsiú a Laidin & a Ngaoilaig.
Stapleton stated that during his missionary work in Ireland he became convinced of the need for a catechism in Irish for the proper religious instruction of the uneducated commons.
He also brought the spelling closer to the pronunciation, e.g. by replacing ⟨thbh⟩ as in uathbhás ('terror') by ⟨f⟩, giving uafás as in modern Irish.
Stapleton sought sanctuary inside St. Patrick's Cathedral upon the Rock of Cashel, County Tipperary, where he was captured by Parliamentarian soldiers under the command of Murrough O'Brien, 1st Earl of Inchiquin, and, with six other priests, he was put to death on the spot.
A letter written in 1656, quoted by Lingard, puts the number at 60,000; as late as 1666 there were 12,000 Irish slaves scattered among the West Indian islands.
"[7] Due to his martyrdom, Pope John Paul II declared him Venerable in 1991 and in 1992 Beatified him, making him Blessed Theobald Stapleton.