27 September 1922: The Passing of the 'Public Safety Act' on this day. It was proposed by the Provisional Government of the Irish Free State. This Act was brought in to allow for the detainment and execution of those found in arms against the State. The Irish Free State was still in the process of formation at this stage but wide sections of the IRA and indeed the general public did not accept its Legitimacy. 

On 28 June of that year the Irish Civil War had broken out in Dublin and quickly spread across the 26 Counties. The War soon degenerated into hit and run raids and sweeps and captures. There were numerous casualties on both sides. But the head of the new Government President WT Cosgrave felt that harsher methods were needed to bring the situation under control. In this he was fully supported by the Minister for Defence General Mulcahy who pressed for its introduction.

It was proposed amongst other measures that if persons were found guilty by Military Courts that they could face the death sentence:
'The breach of any general order or regulation made by the Army Authorities and the infliction by such Military Courts or Committees of the punishment of death or of penal servitude for any period'

Cosgrave spoke that:
If murderous attacks take place, those who persist in those murderous attacks must learn that they have got to pay the penalty for them…They must be taught that this Government is not going to suffer their soldiers to be maimed and ruined, crippled and killed, without at least bringing those responsible for such destruction before a tribunal that will deal out justice to those people.

The Labour Party Leader Thomas Johnson opposed the Bill likening it to a military dictatorship:
We are pretending to govern through this Dáil. We are supposed to have a Government which is responsible to this Dáil. The Government hands over that responsibility to an Army which is not fitted for this particular kind of work—entirely unfitted for this particular kind of work.
http://oireachtasdebates.

oireachtas.ie/debates

Thus it came about that on 27 September a Bill was put before the new parliament that was passed by 41 votes to 18 votes to allow its implementation. 

So Notwithstanding the misgivings of some the Bill was passed and after an Amnesty ran out the first executions took place in November. By the time the Civil war was over 77 men had been executed under its terms.