He was the son of Robert Callaghan and Mary Callaghan. In 1901 the family were living in house 51, Loughgall Town, Loughgall, Co. Armagh. By 1911 the property was known as house 53, Loughgall Town, Loughgall, Co. Armagh.
On 28th September 1912, Callaghan signed the Ulster Covenant at Loughgall giving his address as Royal School, Armagh.
Callaghan would have joined the Belfast Banking Company sometime around 1914 working in Donegal.
Callaghan served with the Royal Inniskilling Fusiliers and attained the rank of 2nd Lieutenant.
The National Archives records a Medal Index Card as follows:
Royal Inniskilling Fusiliers attached Royal Army Service Corps, 2nd Lieutenant
Following his demobilisation after the Great War, Callaghan returned to work.
R M Callaghan was a member of the Belfast Banking Company Sports Club in March 1948 paying 1 shillings subscription and was recorded as being in Banbridge branch.
Retirement came in 1956.
He died on 2nd September 1959.