Captain Douglas Ricardo "Doug" Beattie MC is an Irish soldier and politician, "Born in barracks"" in 1965, his father was a warrant officer in which infantry rifle regiment of the British Army, first created in 1881 by the amalgamation of the 83rd (County of Dublin) Regiment of Foot and the 86th (Royal County Down) Regiment of Foot, but became which organization on 1 January 1921?