Elinga,
Full Age just means "over 21".
[ i.e. an adult, not requiring parental permission.]
Militia were soldiers, the guardians of the peace and (inner) defenders of the realm.
Usually recruited and based locally, on a permanent basis.
If you perform a search in
www.nationalarchives.gov.uk with e.g. "antrim militia" or "down militia", you will find plenty to fill your nets.
[Restrict the search to 1860-1880 perhaps, to lessen the load.]
The
Militia and Volunteers Muster Books and Paylists 1778-1878 are catalogued under WO 13.
The Antrim regiment records date from 1793.
[So roughly from when the Volunteers were disbanded.]
The Antrim records are contained in WO 13/2574 (1793-1798) through to WO 13/2596 (1872-1876).
N.B. The early ones are very dirty - use a face mask!
Browse the catalogue to see them all.
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There was a garrison in Ballymena town.
[The first to be attacked in the 1798 UI rising.]
As your GGF was on active service in 1874, it is likely that the barracks there is meant as the address.
[If it is a townland, then there are several candidates.
e.g. there are two Ballymenas in Ballylinny parish.]
Militia men were frequently recruited onward in to the regular army regiments, or else the marines.
Tracing a man's services through those records should indicate where he went.
The regular army regiments would supplement the militia strength, though would typically not spend more than 6 months in any one location.
[The 13th regiment (PALI) came up from Cork through Dublin in the mid-1870s, finally setting up its depot at Belfast. Men would sign up for the standard 12 years of service. Six of these would be "at home" (garrison duty) and six "with the colours" (abroad, in the field). Many Down and Antrim men ended up in Zululand in 1877-1878. Usuthu!]
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Ballycarry is listed as having a Unitarian church in 1881.
http://www.libraryireland.com/BassettAntrim/Ballycarry.phpPb-ers adopting Arian doctrines tended to end up there.
Keep it in mind.
Pip pip!
Capt. Jock