The history of the Regiment is traced back to 20th June 1689 with raising of Tiffins's Regiment of Foot (later to become the 27th or Inniskilling Regiment of Foot) which fought in the Williamite Wars. Of the remaining Regimental antecedents, the 83rd, 86th, 87th and 89th Regiments of Foot were all raised in 1793 for service in the Napoleonic Wars; the 108th Regiment was taken onto the Army establishment from the East India Company's Army in 1861 following the Indian Mutiny.

On 1st July 1881, the Cardwell Reforms brought about the amalgamations of the 27th and 108th to form the Royal Inniskilling Fusiliers, the 83rd and 86th to form the Royal Irish (later Ulster) Rifles, and the 87th and 89th to form Princess Victoria's (Royal Irish Fusiliers). On 1st July 1968 these three Regiments amalgamated again to form The Royal Irish Rangers (27th (Inniskilling), 83rd and 87th).

The Ulster Defence Regiment was raised in April 1970 to protect the Border and the State against sabotage and armed attack in Northern Ireland. On 1st July 1992 The Royal Irish Rangers, the last Irish Infantry Regiment of the line, merged with The Ulster Defence Regiment to form The Royal Irish Regiment. The Royal Irish Regiment is unique in the British Army. It consists of one Air Assault battalion (1 R IRISH), liable for service world wide, and one Reserve battalion (2 R IRISH) who are part-time soldiers based in Northern Ireland and provide our Reserve when required.  2 R IRISH are an integral part of our ability to deploy in the world anywhere, at short notice, and provide a capable fighting force.