In 1586, Collins joined a cavalry unit during the Brittany Campaign of the French Wars of Religion; namely, the Catholic League led by Philippe Emmanuel, Duke of Mercœur.

Collins' other enemy was the House of Bourbon, as the Duke de Mercoeur also sought, with both Spanish and Vatican backing, to restore the political independence of the Duchy of Brittany from the Kingdom of France.

[8] Henri of Navarre, the French throne's still Calvinist heir presumptive, offered Collins a bribe of 2,000 ducats to surrender the chateau and surrounding district, but was refused.

White, who had previously founded the Irish College at Salamanca, has left behind a biography of Dominic Collins, which relates in considerable detail the story of the former Captain's call to the religious life.

He began his novitiate in the Jesuit college at Santiago de Compostela, where the annals describe Collins as, "an Irishman of distinguished parentage, comely appearance and stature", who had just, "passed thirty-two years of age", and, "had been a Captain of Duke Philip's cavalry in Brittany.

[5] Soon after his profession, a Spanish expedition was organised by King Philip III to assist the rising of the Irish clans led by Aodh Mór Ó Néill and Red Hugh O'Donnell during the Nine Years War against Queen Elizabeth I and in resistance to the Reformation in Ireland.

Seeking to rescue Juan del Águila's forces and break the siege, the anti-Tudor coalition of the Irish clans of Ulster under the leadership of Aodh Mór Ó Néill and Red Hugh O'Donnell marched across the whole length of Ireland through extremely bitter winter conditions, but were catastrophically defeated by the English cavalry at the Battle of Kinsale on 24 December 1601.

"[12] Determined to continue the fight, Archer and Collins left Kinsale and joined the forces of Donal Cam O'Sullivan Beare, who were still holding out with Spanish backing further down the coast at Dunboy Castle.

[13] In a letter to Robert Cecil, Sir George Carew, "could not forbear", according to Irish historian Proinsias Ó Fionnagáin, "voicing his almost superstitious fear of Archer".

Archer returned to Spain to seek further reinforcements, while Dominic Collins remained behind at Dunboy Castle with 143 Irish soldiers who were besieged by 4,000 English troops.

Determined upon the propaganda coup of having a Jesuit abjure the Catholic Faith in his very hometown, the Queen's officials removed Collins from Shandon Castle and brought him back to Youghal, a distance of 173 km.

[17] According to Pacata Hibernia, a detailed account of Sir George Carew's term as President of Munster, which was compiled under his close supervision soon after the events it describes and published posthumously in 1633,[14] "the Fryer, in whom no penitence appeared for his detestable treasons, nor yet would endeavour to merit his life, either by discovering the Rebells' intention, (which was in his power) or by doing some service that might deserve favour, was hanged at Youghall, the Towne wherein he was born.

"[18] The accounts by Bishop David Rothe and John Mullin confirm that, after being tried and condemned to death by military tribunal,[19] Dominic Collins was publicly hanged at Youghal on 31 October 1602.

When he arrived at the foot of the gallows, he fell on his knees and kissed it, commending his passage to God; then, following the example of the martyrs, he prayed for his enemies, for the Queen, and for his country, and with alacrity and a cheerful countenance ascended the ladder.

Collins's last words before being turned off of the ladder and hanged were, "Look up to heaven and, worthy descendants of your ancestors, who ever constantly professed it, hold fast to the faith for which I am this day to die!

[23] Collins was beatified – along with Francis Taylor, Dermot O'Hurley, Margaret Ball, and 13 other Irish Catholic Martyrs – on 27 September 1992 by Pope John-Paul II.