Question: Denis O'Brien was a veteran of what conflict that led to the establishment of the Irish Free State?

Context paragraphs:
Courts of Justice Act 1924: The Courts of Justice Act, 1924 was an Act of the Oireachtas (No. 10 of 1924) that established a new system of courts for the Irish Free State (now the Republic of Ireland).  Among the new courts was the Supreme Court of the Irish Free State, and the first Chief Justice of the Irish Free State was also appointed under the Act.

Irish Civil War: The Irish Civil War (Irish: "Cogadh Cathartha na hÉireann" ; 28 June 1922 – 24 May 1923) was a conflict that followed the Irish War of Independence and accompanied the establishment of the Irish Free State, an entity independent from the United Kingdom but within the British Empire.

Irish Boundary Commission: The Irish Boundary Commission (Irish: "Coimisiún na Teorainne" ) met in 1924–25 to decide on the precise delineation of the border between the Irish Free State and Northern Ireland.  The 1921 Anglo-Irish Treaty, which ended the War of Independence, provided for such a commission if Northern Ireland chose to secede from the Irish Free State, an event that occurred as expected two days after the Free State's inception on 6 December 1922.  The governments of the United Kingdom, of the Irish Free State and of Northern Ireland were to nominate one member each to the commission.  When the Northern government refused to cooperate, the British government assigned a Belfast newspaper editor to represent Northern Irish interests.

Enactment of the Constitution of Ireland: The current Constitution of Ireland came into effect on 29 December 1937, having been enacted by a national plebiscite on 1 July 1937 of voters in what was then the Irish Free State.  The Constitution was closely associated with Éamon de Valera, the President of the Executive Council of the Irish Free State at the time, who was personally eager to replace the Constitution of the Irish Free State.

Seanad Éireann (Irish Free State): Seanad Éireann (] ; "Senate of Ireland") was the upper house of the Oireachtas (parliament) of the Irish Free State from 1922 to 1936.  It has also been known simply as the Senate, First Seanad, Free State Senate or Free State Seanad.  The Senate was established under the 1922 Constitution of the Irish Free State but a number of constitutional amendments were subsequently made to change the manner of its election and its powers.  It was eventually abolished in 1936 when it attempted to obstruct constitutional reforms favoured by the government.  It sat, like its modern successor, in Leinster House.

Irish Republican Army (1922–69): The original Irish Republican Army (IRA) fought a guerrilla war against British rule in Ireland in the Irish War of Independence between 1919 and 1921.  Following the signing of the Anglo-Irish Treaty on 6 December 1921, the IRA in the 26 counties that were to become the Irish Free State split between supporters and opponents of the Treaty.  The anti-Treatyites, sometimes referred to by Free State forces as Irregulars, continued to use the name Irish Republican Army (IRA) or in Irish Óglaigh na hÉireann, as did the organisation in Northern Ireland which originally supported the pro-Treaty side (if not the Treaty). " Óglaigh na hÉireann" was also adopted as the name of the pro-Treaty National Army, and remains the official legal title of the Irish Defence Forces.  This article deals with the anti-Treaty IRA that fought against the Irish Free State in the Irish Civil War, and with its successors up to 1969, when the IRA split again.

Denis O'Brien (police officer): Detective Sergeant Denis O'Brien, Registration Number: 8288 (17 June 1899 – 9 September 1942), often called "Dinny O’Brien", was a veteran of the Irish War of Independence and the Irish Civil War.  He joined the Garda Síochána in 1933 and was killed by the Anti-Treaty IRA.

Constitution of the Irish Free State: The Constitution of the Irish Free State (Irish: "Bunreacht Shaorstáit Eireann" ) was the second constitution of the independent Irish state.  It was adopted by Act of Dáil Éireann sitting as a constituent assembly on 25 October 1922.  In accordance with Article 83 of the Constitution, the Irish Free State Constitution Act 1922 of the British Parliament, which came into effect upon receiving the royal assent on 5 December 1922, provided that the Constitution would come into effect upon the issue of a Royal Proclamation, which was done on 6 December 1922.  In 1937 the Constitution of the Irish Free State was replaced by the modern Constitution of Ireland following a referendum.

Irish constitutional referendum, 1937: The Constitution of Ireland plebiscite, also known as the Plebiscite on the Draft Constitution, was a plebiscite held in the Irish Free State to approve the adoption of a new constitution for the country.  The vote was held on 1 July 1937, the same day as the general election.  It was passed by a plurality, with 56% of voters in favour, comprising 38.6% of the whole electorate.  As a result, the current Constitution of Ireland came into effect on 29 December 1937, ending the Irish Free State.  It was the only plebiscite to take place in the Irish Free State.  It was also the only referendum that was officially called a plebiscite in the history of the country; all subsequent votes of this nature have taken place under the provisions of the current Irish constitution and are referred to as referendums.

Comhairle na dTeachtaí: Comhairle na dTeachtaí was an Irish republican parliament established by opponents of the 1921 Anglo-Irish Treaty and the resulting Irish Free State, and viewed by republican legitimatists as a successor to the Second Dáil.  Members were abstentionist from the Third Dáil established by the pro-Treaty faction.  Just as the First Dáil established a parallel Irish Republic in opposition to the British Dublin Castle administration, so Comhairle na dTeachtaí attempted to establish a legitimatist government in opposition to the Provisional Government and Government of the Irish Free State established by the Third Dáil.  This legitimatist government, called the Council of State, had Éamon de Valera as President.  In 1926 de Valera resigned as President, left the Sinn Féin party and founded Fianna Fáil, which in 1927 entered the Fourth Dáil.  Comhairle na dTeachtaí, never more than a symbolic body, was thereby rendered defunct.  In 1930 Cumann na nGaedheal TDs alleged in the Dáil that de Valera had addressed Comhairle na dTeachtaí in December 1926, after the foundation of Fianna Fáil; this was to cast aspersions on de Valera's commitment to the Constitution of the Irish Free State.

Supporting paragraphs: Denis O'Brien (police officer) Irish Civil War
Answer: Irish Civil War