He was educated at Congregation of Christian Brothers schools and then completed his seminary studies at St Patrick's College, Maynooth, where he was ordained as a priest.

Mannix was president of St Patrick's College, Maynooth, the Irish national seminary, from 13 October 1903 to 10 August 1912 when he was succeeded by the Rt Reverend John F. Hogan.

During his presidency, he welcomed both Edward VII in 1903 and George V in 1911 with loyal displays, which attracted criticism by supporters of the Irish Home Rule movement.

[2] Mannix was also involved in the controversy surrounding the dismissal of Father Michael O'Hickey[3] as Professor of Irish after O'Hickey publicly attacked those members of the senate of the National University of Ireland who opposed making Irish a compulsory subject for matriculation and insinuated that the senators (who included several bishops) had sinned grievously by so doing and resembled those MPs who were bribed to pass the Act of Union.

In Australia at this time, the Irish Catholics were commonly treated with disdain by the English and Scottish majority (who were mostly Anglicans and Presbyterians respectively) and also as potentially disloyal.

[citation needed] He became, however, increasingly radicalised and in October 1920 led an Irish republican funeral cortège through the streets of London following the death of the hunger striker Terence MacSwiney, the Lord Mayor of Cork City in Mannix's native county.

In 1920, Mannix travelled from Melbourne to San Francisco and then by train he journeyed to New York in order to take passage on the White Star Line ship the RMS Baltic to Ireland.

This show of support was to send off Mannix, who had been so outspoken on the English rule in Ireland, and successfully led anti-conscription campaigns during World War I.

[6] However shortly before the RMS Baltic was due to arrive in Cork Harbour, it was stopped and boarded by British military, who arrested Mannix and transferred him directly to England.

[7][8] By the end of the war, Mannix was the recognised leader of the Irish community in Australia, idolised by Catholics but detested by others, including those in power federally and in Victoria.

[citation needed] Mannix formed the Irish Relief Fund, which provided financial support for the families of those shot or imprisoned by the British.

From his palatial house, "Raheen", in Kew, Melbourne, he would daily walk to and from St Patrick's Cathedral, personally greeting any of his flock that he encountered.

In 1920 he led large St Patrick's Day parade with a guard of honour made up of Irish Australian winners of the Victoria Cross.

[2] Another associate was William Hackett SJ, a Jesuit priest from Ireland, who had been involved in the Irish Republic's struggle for independence from Britain before being posted to Australia.

[9] Mannix surprised many of his supporters by opposing this, on the grounds that the bill was totalitarianism, which in his view was worse than communism: his may have been a decisive influence in the referendum's narrow defeat.