Following the Territorial Forces Act 1908, the Militia battalions of British army infantry regiments were converted to Special Reserve battalions and typically numbered 3rd & 4th battalions after their two line battalions. There was never routine photographing of recruits or trained soldiers of the Militia or indeed Regular army until much later in the 20th Century.
The 6th (Service) Battalion Royal Irish Regiment was one of several hostilities only battalions that were created during the First World War.
After the partition of Ireland and the creation of the Irish Free State in 1922, along with 5 other Irish regiments, the Royal Irish Regiment was disbanded. I don’t know of any records in Ireland and believe ALL records were transferred to the England and surviving ones were eventually deposited in the National Archives, Kew.
Surviving records, such as they are, of men who served in WWI including any from their pre-war service, are available at the National Archives, Kew and are also available through a licensing arrangement on Ancestry. Records up to 1915, generally attestation and/or discharge papers, from National Archives documents series WO 97 (regular army) and WO96 (militia) are now also available online via another 3rd party pay per view site.