I think if you are considering living there, you really should visit the place before you commit to live there.
Walk around the town, visit the local shops, and speak to the shopkeepers, ask the locals directions, to see the response to your Mayo accent.
You will then have a personal opinion of the place.
While the internet is great for getting info for holidays etc, it is quite different to living in an area.
If you have never lived in Northern Ireland, you may not like it.
When i lived there, i learned quickly that people were working out my religion by asking a few simple questions: surname, what school I went to, Derry or Londonderry, did i follow rugby or GAA (strange I know)?
If it is important to live in a 'Catholic' area, Derry city is regarded as a 'Catholic' city. The Waterside of the city and around the Fountain would be the main 'Protestant' areas, if i can remember correctly.
In my opinion, if you are so concerned about this, you really should consider living in Donegal.
The locals might just enjoy the banter about the GAA. e.g. Mayo's and Donegal's recent performances in the championships
!!!
Finally, Donegal people are very chatty overall, you would have no problems settling in!