Welcome to Rootschat.
IGI lists 4 children to Daniel Friar & Mary McBride:
1. Catherine born 3 March 1867 Letterkenny district.
2. Mary born 29 Oct.1869 Co. Donegal.
3. John born 25 June 1872 Co. Donegal.
4. Dennis born 28 Dec.1875 Co. Donegal.
There could be other children simply not in IGI (many but not all Irish births 1864-c1882 are there). 1864 was the start of civil registration of births so any older children will not have birth certificates.
Do you have John's birth certificate? IGI entry lists his birthplace as Co, Donegal not Letterkenny. Catherine's brth is listed under Letterkenny district but she may have been born in a townland within that registration district and not Letterkenny itself.
Don't understand this part of your posting:
I have found an IGI record of them on the family search.org website however they were R.C so assume they changed to suit a lifstyle.