Brennan joined the Irish Volunteers in 1917, was appointed Battalion Commandant of 4 Battalion, North Wexford Brigade and was involved in raids and other armed operations
during Irish War of Independence.[2] Taking the anti-Treaty side in the Irish Civil War (1922-1923), Brennan took part in fighting against National forces in County Wexford and County Wicklow. He was captured on 28 July 1922, took part in the burning of Portlaoghise Jail, underwent 23 days hunger strike, and was released on 11 May 1924.[3] Brennan later applied to the Irish government for a service pension under the Military Service Pensions Act, 1934 and was awarded 5 and 2/3 years service at Grade C for his service with the Irish Volunteers and the IRA between 1 April 1918 and 30 September 1923. [4]
^ See Brennan's successful application for a military service pension under the Military Service Pensions Act, 1934. Available online at Military Service (1916-1923) Pensions Collection - http://mspcsearch.militaryarchives.ie/search.aspx?formtype=advanced. Reference number MSP34REF36218