As regards the executed 1916 leaders. They were adjudged to have rebelled against His Majesty's government in a time of war. As such, they were deemed to guilty of 'treason', a capital offence.

In all, 97 of those involved in the Easter Rising were due to face the firing squad. In fact, in Arbour Hill, the mass-grave into which the remains of the 14 men executed at Kilmainham Gaol were buried could have held up to 68 to 70 bodies. Generals Maxwell and Blackadder had not intention of limiting the executions to just 14.

ON the very day that Seán MaDiarmada and James Connolly were executed, the British Prime Minister Herbert Asquith arrived in Dublin to see General Maxwell and he carried a letter from King George V demanding that General Maxwell 'end this madness'. As a result, the executions stopped with MacDiarmada and Connolly.
Thomas Kent had been executed in Cork already.
Casement, of course was a different matter and he was tried under civil law and then hanged at Pentonville Prison on Aug 3rd, 1916.

The rest of the leaders due to be executed including Dev, William Cosgrave, Piaras Beaslaí and the others all received commuted sentences from life right down 5 and 3 yrs.

All, of course were released within a year as part of a British government amnesty.

* Amazing what you learn while working at Kilmainham Gaol*!!