William James Brennan-Whitmore was born in Wexford, Ireland on 21 October 1886 to Thomas Whitmore, a journalist and painter, and his wife Mary Brennan.

[1] Serving in the Royal Irish Rifles, Brennan-Whitmore was deployed to the British Raj in the medical corps and at one point received a promotion to the rank of sergeant.

[2] Brennan-Whitmore left the army in 1907 and returned to his home in County Wexford, becoming a journalist and a prominent member of local branches of the Gaelic League and Sinn Féin.

Retreating under heavy British bombardment as his command post and other buildings in the vicinity were set ablaze by shelling, Brennan-Whitmore and the group under his leadership became lost due to his lack of knowledge of the city of Dublin's layout.

[1] He became closely associated with Michael Collins and Richard Mulcahy and lectured rebel officers on guerrilla warfare while also frequently challenged the British authorities to provide prisoner-of-war privileges to the Irish republican prisoners, efforts which often resulted in success.

Brennan-Whitmore supported the signing of the Anglo-Irish Treaty and was appointed as a commandant in the National Army, adjutant in Wexford and intelligence officer for Carlow in 1922.

[1] In his new residence of Gorey, Brennan-Whitmore established a local newspaper, the Record and a printing business named the Arcadian Press, through which he called for the replacement of parliamentary democracy with a corporatist state.

[1] Following the outbreak of World War II he wrote letter to the press expressing pro-Axis views and founded the Celtic Confederation of Occupational Guilds, a short-lived organisation which along with its successor Saoirse Gaedheal achieved little success despite being the recipient of funding from the Abwehr.

[9] Despite his far-right views, Brennan-Whitmore became an active member of the left-leaning Clann na Poblachta, hoping that in government the new party would introduce financial reforms and break the de facto parity between Irish pound and the Sterling.