The Royal Irish, as it will be known, combines the Royal Irish Rangers and the Ulster Defence Regiment, and will recruit across Ireland, north and south.
It is the first of the new regiments created by the Government's Options for Change defence plans, but it bears an ancient name, recalling Ireland's long military tradition. The first regiment to bear that name was raised in 1684. The 36th Ulster Division and 16th Irish Division were among those launched into the wasteful first Battle of the Somme in July, 1916.
The merger brings the UDR into the regular British Army structure for the first time. Initially, the former Rangers will dominate the senior ranks.
There was concern that the 97 per cent Protestant composition of the UDR and the presence of certain 'bad apples' made it a threat to the Catholic community. Bringing those regiments fully into the British Army should give more control over them.
The Royal Irish Rangers were 24 per cent Roman Catholic, with 6 per cent of their soldiers from the Irish Republic. The merger will clearly dilute the Catholic component. But the new regiment is keen to recruit from all of Ireland and Irishmen on the mainland. A recruiting campaign in the Republic has had some success, as the British Army offers more opportunities than does the Irish Army.
There are seven 'home service battalions', based on the UDR, which was formed to fight terrorism in the Province, and two 'general service' or overseas battalions, the former Royal Irish Rangers, which will reduce to one next year. The home service battalions will contain full-time and part-time members. The regiment's first overseas battalion will deploy to Cyprus next month.
On the eve of the merger, the UDR had 5,900 soldiers, the Rangers 1,100. The final strength of the new regiment, still to be confirmed, will be about 6,000. The colonel in chief will be the Duke of York. 'He hasn't quite got 10,000 men but it's not far off,' a senior Royal Irish officer said.
When the merger was announced on 23 July last year, it came as a surprise. 'Everybody was a bit suspicious at first,' a colonel in the new regiment said.
But the advantages were apparent. 'It gave us the chance to bring the UDR fully into the regular Army. Also, the Royal Irish Regiment secured the future of Irish infantry.'
The new regiment will use the former Rangers' training centre at Ballymena, Co Antrim, with facilities at Ballykinler, Co Down, and Magilligan, Co Londonderry.