Was John Shields snr from Antrim? If so, FindMyPast has this service record with image:
British Army Service Records collection
Name: John Shields
Location: Antrim, Ireland
No birth or death date listed in the search result
The soldier above possibly served with the 18th Royal Irish - one of the regiments posted to Jersey c1813.
Hopefully you have access to FindMyPast to view this record. I've no idea what year(s) it represents.
The record might also be on Ancestry in the UK, Royal Hospital Chelsea Pensioner Admissions and Discharges, 1715-1925 collection, but you'd need Fold3 access to view it.