There was no birthplace prerequisite for being a member of either a pre-partition Irish or indeed a Scottish regiment as they were all part of the British Army.
Unlike for the majority of British army regiments, whose pre-c1923 service records are held at the National Archives, Kew, records of former members of the Foot Guards Regiments are held by their respective regimental HQs. Although it is possible to telephone the regimental archivist, written applications are preferred; address is:
Irish Guards RHQ
London SW1E 6HQ