Have you found his Army records? You can find a link to a site that carries the National Archives
British Army records from Kew. It is a pay website.
1823 is very early for Irish birth records for many reasons. Was he CofI or RC?
There are two bmd websites for Fermanaghwww.emeraldancestors.comwww.rootsireland.ie
The Tithe Applotments are the earliest census substitute 1823-1837, however, either there are
no Tithes for Fermanagh or no Rankin's in Fermanagh.
The Griffith Valuations 1846-1864, and 1862 in Fermanagh, a lot of years between.
There is a one 1 Rankin in Enniskillen Parish, Enniskillen Townland and Enniskillen Poor Law Union
Johnston Rankin Parish of Enniskillen, TL Enniskillen & PLU Enniskillen
tenant of Edward H. Innes
#17 Market St. a house, outbuilding and yard (no acreage)
However, if you look at the Rankin's in the Poor Law Union of Enniskillen but in different parishes than Johnston Rankin, there are several. A PLU covers an administritive area of 12 mi. and contains parishes, villages and towns within 12 mi.
These are the names. Adam (very unusual) Rankin; David (unusual) Rankin; George (not common but not rare)Rankin'; Dorothy and Mary Jane who maybe widows as rarely is property leases in women's names. Are any of these names used in your Nathaniel Rankin family?
I have two UK census that identify Irish born:
1871: #10 Workhouse Lane, Cove Rd. Alexander Beer House, Farnborough, Hampshire
Nathaniel Rankin age 46, Chelsea Pensioner born Co. Fermanagh
Mary 31, no place of birth listed?
William age 12 born Dublin Ireland
Edith B. age 4 born Hants, Eng.
Nathaniel age 2, born Hants, Eng
Marie M. 3 mo. born Hants, England
Next you should try the UK census for 1881, and 1901, 1911, there would be probably lots of Mary's so follow Nathaniel and the girls.
Dublin births: www.irishgenealogy.ie
The baptism of William Rankin was not in 1869 (est: birth year) but on January 9, 1859, this is not a transcription error as you get to see the digitized parish register records.
He was Baptised in the Military Church, Arbour Hill, Dubl, Fa: Nathaniel Sgt. 76th. Regiment
and Mo. Mary Jane Date: January 9, 1859
Elizabeth Constance born 2 Sept 1860, baptised 30 Sept 1860, Richmond Barracks, Dublin
Fa: Nathaniel Rankin 76th Foot, mo: Mary
Although I have never seen it elsewhere but in Ireland if a son dies, a later son would be given the same forename. Its quite unusual but definitely not uncommon in Ireland..