Active in Cork and in Dublin, they opposed a Protestant faction in the leadership who, fearing that the British Crown could buy loyalty through offers of emancipation, mistrusted Catholic intentions.

Henry attended Trinity College Dublin, bought an officer's commission and then studied as a lawyer, being called to the bar as a barrister in Michaelmas term, 1790.

[2] On the boat from France to England they met a young Daniel O'Connell to whom they displayed a handkerchief soaked, they claimed, in the blood of Louis XVI, the late executed king.

He was opposed by a faction led by William Drennan who feared that, rather than commit to a democratic union with Presbyterians and other disaffected Protestants, Catholic leaders would continue to court the government in the hope of further concessions.

[6] From the beginning of 1794, the Crown was at war with the new French Republic, and in a succession of steps presaging martial law in Ireland, the prospects for reform rapidly receded.

While two other less famous brothers enlisted in the British army (and were later killed), and while continuing with their own legal careers, Henry and John Sheares enthusiastically engaged in the work of the United Irish societies as they transformed themselves from political clubs into an insurrectionary movement.

[8] As government successes, including the arrest in Dublin of the Leinster provincial committee in March 1798, increased pressure on the leaders at large to initiate action in advance of the hoped-for French assistance, the Sheares brothers found themselves isolated.

[9] Already quietly betrayed by Conway and Collins, John also befriended Captain Warnesford Armstrong from County Down, who claimed to be a busy member of the party there.

Their lawyer was John Philpot Curran who, with Sir Jonah Barrington, obtained a stay of execution in the hope that Henry would recant, but the brothers were already dead.