[2] He decided on a vocation of consecrated life and was admitted to minor orders in June 1618, before becoming ordained to the Catholic priesthood the following year in December.

[2] After standing down from public teaching in universities, he authored and published his most notable text at Lisbon in 1645: Disputatio apologetica de iure regni Hiberniae pro catholicis Hibernis adversus haereticos Anglos ("An Argument Defending the Right of the Kingdom of Ireland for Irish Catholics against English Heretics").

[3] The major areas he measures English rule up against from a perspective of Catholic moral teaching are: just war, religious mission, consent and peaceful possession.

O'Mahony argued that Henry II of England's invasion of Ireland was "evil" because it sought to restore an adulterer, Diarmaid mac Murchadha, to his throne.

"[3] He also argued that Laudabiliter had been obtained by the Normans under "false pretences" in the first place, but whatever the argument may have been, now that England had embraced Protestantism (a heresy in the eyes of the Catholic Church), including its last three monarchs — Elizabeth I, James I and Charles I— its claims to "religious mission" in Ireland was now totally null and void.

[3][2] In formulating these arguments he drew on the works of Robert Bellarmine, Francisco Suárez and Luis de Molina.

[3] Although the work was intended as a boost to the radical wing of the Catholic Confederacy; that of Owen Roe O'Neill and Cardinal Giovanni Battista Rinuccini, contrary to the more "moderate", compromising, pro-Stuart Old English Irish Catholic royalists who wanted to make peace with the Marquess of Ormond; it became a taboo and banned book.

In general, the Confederates wanted to distance themselves from some killings of Protestant civilians which took place earlier in 1641 and also, leave the door open to at least the possibility of the rights of Catholics being restored under a Stuart monarchy.

Nevertheless, Sir Kenelm Digby, an English Catholic diplomat, who represented Charles I in Rome, complained that Cardinal Rinuccini tolerated the Disputatio apologetica (he had refused to hand Athlone priest Fr.

John Bane over to the authorities after he was found with a copy) and for the Old English within the Irish Confederacy they feared that the book was on the backburner to endorse making Owen Roe O'Neill the High King of Ireland in place of the Stuart dynasty if the opportunity availed itself.

[3][2] The "go-to" reference work for the Irish view of the period has tended to have been the six-volume Commentarius Rinuccinianus compiled by Fr.