Greetings,
I am relatively new to Irish genealogy and wondering if I could have some advice. This situation is, perhaps, quite unique, but I would be grateful for any help.
My ancestor John DAVIS, who was born in England, was a soldier with the 81st Regiment of Foot from the late 1840s until the mid 1870s. The 18st Regiment was shipped to Ireland for garrison duty in the early 1850s in which they were to depart for India. They stopped in Fermoy for a month where John married one Jane SHERLOCK, the daughter of Peter Sherlock, on 18 June 1853. As they were Protestant their marriage is Indexed and stored in the Irish Registrar Office in Dublin.
There was another Sherlock girl, Mary, who married in 1848 which was also a Protestant marriage and registered in Fermoy
district (not necessarily Fermoy itself). I do not know the relation if any.
However, when John was finished with his military duty he returned to England and settled in Surrey. On the 1881 England Census Jane's birth location is recorded as Saint Helena island in the Atlantic. I have searched the Saint Helena birth registrations for the time period and have not found any Sherlocks.
John does not appear to have married a second time (to another woman named Jane), but that is possible. I know that Fermoy was a military garrison at the time, and it is possible that Peter Sherlock, Jane's father, was also in the military and had his daughter on Saint Helena, also a military base.
-> Should I request a full certificate from the Ireland General Registrar Office? This should list Jane's father Peter's occupation to see if there's a military connection.
-> Does anyone know of a Sherlock family or families from the Fermoy area?
-> I believe the church John and Jane were married in is called Christchurch. Is it worth sending a letter to the current vicar? Do you typically get replies from Irish churches by using this method, or is it better to potentially find a native genealogist who can visit in person.
-> Since it is possible that Jane is not an Irishwoman at all, and both families are military, is there any specific resources anyone knows of for military garrisons in Ireland, or sources in general that I am not thinking of for military families of UK soldiers in Ireland?
Thank you for your help in advance. It is a giant puzzle I am trying to solve. I have so many places: Gloucestershire, Fermoy, Co. Cork, Saint Helena, India... Gibraltar... etc. Any help would be vastly appreciated from those of you with a lot more Irish genealogy experience than myself.

Best regards,
Mike