Question: 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?

Documents:
source: Doug Beattie
text: Doug Beattie Captain Douglas Ricardo "Doug" Beattie MC is an Irish soldier and politician. " "Born in barracks"" in 1965, his father was a warrant officer in the Royal Ulster Rifles. The family settled in Portadown when he was 10 following the cessation of his father's regular service but saw Beattie Snr enlist into the Ulster Defence Regiment shortly after.

source: Royal Ulster Rifles
text: Royal Ulster Rifles The Royal Irish Rifles (became the Royal Ulster Rifles from 1 January 1921) was an 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. The regiment saw service in the Second Boer War, the First World War, the Second World War and the Korean War.

source: 11th Mechanized Corps (Soviet Union)
text: 11th Mechanized Corps (Soviet Union) The 11th Mechanized Corps was formed from March to September 1932 from the 11th Rifle Division in Leningrad, one of the first two Red Army mechanized corps. The corps was commanded by division commander Komkor Kasyan Chaykovsky and its chief of staff was Mikhail Bakshi. The 31st Mechanized Brigade was formed from the 32nd Rifle Regiment named for Volodarsky, the 32nd Mechanized Brigade from the 33rd Rifle Regiment named for Voskov, the 33rd Rifle and Machine Gun Brigade from the 31st Rifle Regiment named for Uritsky. The 31st Brigade was equipped with the T-26 and the 32nd Brigade was equipped with the BT-2. The corps at the time had a total of 220 tanks. On 1 January 1933 the 83rd Aviation Group was attached the corps, and was later reformed into the Motor-Mechanized Squadron. By March of that year the brigades were based in Tsarskoye Selo, Slutsk, and Stary Peterhof, while the corps headquarters and rear units were still in Leningrad. In December, the 32nd Brigade's 1st Tank Battalion was transferred to the 6th Mechanized Brigade in the Special Red Banner Far Eastern Army. On 16 January 1934 the corps received the honorific "Leningrad", the 31st Brigade received the honorific "named for Uritsky", the 32nd Brigade the honorific "named for Volodarsky", and the 33rd Brigade the honorific "named for Voskov".

source: Cameronians (Scottish Rifles)
text: Cameronians (Scottish Rifles) The Cameronians (Scottish Rifles) was a rifle regiment of the British Army, the only regiment of rifles amongst the Scottish regiments of infantry. It was formed in 1881 under the Childers Reforms by the amalgamation of the 26th Cameronian Regiment and the 90th Perthshire Light Infantry. In 1968, when reductions were required, the regiment chose to be disbanded rather than amalgamated with another regiment, one of only two infantry regiments in the British Army to do so, with the other being the York and Lancaster Regiment. It can trace its roots to that of the Cameronians, later the 26th of Foot, who were raised in 1689. The 1881 amalgamation coincided with the Cameronian's selection to become the new Scottish Rifles.

source: East Lancashire Regiment
text: East Lancashire Regiment The East Lancashire Regiment was, from 1881 to 1958, a line infantry regiment of the British Army. The regiment was formed in 1881 under the Childers Reforms by the amalgamation of the 30th (Cambridgeshire) Regiment of Foot and 59th (2nd Nottinghamshire) Regiment of Foot with the militia and rifle volunteer units of eastern Lancashire. In 1958 the regiment was amalgamated with the South Lancashire Regiment to form the Lancashire Regiment which was, in 1970, merged with the Loyal Regiment (North Lancashire) to form the Queen's Lancashire Regiment. However, in 2006, the Queen's Lancashire were amalgamated with the King's Own Royal Border Regiment and the King's Regiment (Liverpool and Manchester) to form the present Duke of Lancaster's Regiment (King's, Lancashire and Border).

source: Queen's Own Royal West Kent Regiment
text: Queen's Own Royal West Kent Regiment The Queen's Own Royal West Kent Regiment was a line infantry regiment of the British Army in existence from 1881 to 1961. The regiment was created on 1 July 1881 as part of the Childers Reforms, originally as the Queen's Own (Royal West Kent Regiment), by the amalgamation of the 50th (Queen's Own) Regiment of Foot and the 97th (The Earl of Ulster's) Regiment of Foot. In January 1921, the regiment was renamed the Royal West Kent Regiment (Queen's Own) and, in April of the same year, was again renamed, this time as the Queen's Own Royal West Kent Regiment.

source: King's Royal Rifle Corps
text: King's Royal Rifle Corps The King's Royal Rifle Corps was an infantry rifle regiment of the British Army that was originally raised in British North America as the Royal American Regiment (also known as the Royal Americans) in the Seven Years' War and for Loyalist service in the American Revolutionary War. Later, ranked as the 60th Regiment of Foot, the regiment served for more than 200 years throughout the British Empire. In 1958, the regiment joined the Oxfordshire and Buckinghamshire Light Infantry and the Rifle Brigade in the Green Jackets Brigade and in 1966 the three regiments were formally amalgamated to become the Royal Green Jackets. The KRRC became the 2nd Battalion Royal Green Jackets. On the disbandment of 1/RGJ in 1992, the RGJ's KRRC battalion was redesignated as 1/RGJ, eventually becoming 2/RIFLES in 2007.

source: Royal Irish Regiment (1992)
text: Royal Irish Regiment (1992) The Royal Irish Regiment (27th (Inniskilling) 83rd and 87th and Ulster Defence Regiment) (R IRISH) is an infantry regiment of the British Army. The regiment was founded in 1992 through the amalgamation of the Royal Irish Rangers and the Ulster Defence Regiment. Their oldest predecessor; the 27th Regiment of Foot; was first raised in June 1689 to fight in the Williamite War in Ireland. Other notable regiments in their lineage include the Royal Inniskilling Fusiliers, Royal Irish Rifles and the Royal Irish Fusiliers (Princess Victoria's).

source: 83rd (County of Dublin) Regiment of Foot
text: 83rd (County of Dublin) Regiment of Foot The 83rd (County of Dublin) Regiment of Foot was a British Army line infantry regiment, which was formed in Ireland in 1793 for service in the French Revolutionary Wars. The regiment served in the West Indies, South Africa and the Peninsular War, and after the end of the wars with France spent much of the nineteenth century in colonial garrisons. Among other service, the 83rd fought in the Ceylon Great Rebellion of 1817–18, the Canadian Rebellions of 1837, and the Indian Rebellion of 1857. Under the Childers Reforms, the regiment amalgamated with the 86th (Royal County Down) Regiment of Foot to form the Royal Irish Rifles in 1881.

source: 86th (Royal County Down) Regiment of Foot
text: 86th (Royal County Down) Regiment of Foot The 86th (Royal County Down) Regiment of Foot was an infantry regiment of the British Army, raised in 1793. Under the Childers Reforms it amalgamated with the 83rd (County of Dublin) Regiment of Foot to form the Royal Irish Rifles in 1881.

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Answer: Royal Ulster Rifles