Gilliland (Gillen)

My gr. grandfather Francis Gillen (b. 1875) was born in Tullycorbet to Terrence Gilliland and Maria Duffy. It took some time to find his family due to the fact I was searching for the Surname Gillen. Through dna matching on 23andme I connected with a Gillen cousin also searching for the Gillen family in Ireland. Turns out her grandfather Terry and my gr. grandfather Frank.
She was able to dig up her grandfather Terry's marriage certificate in which he signed his name Terrence Gilliland with his mother listed as Maria Duffy and father as Terrence. My quest is to find out why the name change?
In the brief research on the Gilliland family, it seems that almost all other Gillilands in the area were Church of Ireland and ours was Catholic. I have found the marriage record (1864) of Terrence and Maria but it does not list a denomination. All of their many children were baptized Catholic. My speculation is that Terrence was Church of Ireland and Maria was Catholic. I will never know unless I can find their birth records which would be from approximately 1840.
I would appreciate some clues from someone knowledgeable in Tullycorbet genealogy.

Regards,
Mike Schweizer