Thank you all so very much for the replies and very good additional information. You have sparked more thoughts on ways to dig!
I apologize for not including my great grandparents names and yes, we have pretty good family info (from a meeting w/ cousins in Garrison in 2004 that my American cousin had) that my grandfather was born at what they call the family home in Garrison and my American cousin visited the family plot at the cemetery there. And my brother directly connected Garrison to his search as well. But I will also look at another location that we have recorded for my great grandparents.
I've searched online church records (as well as the other resources) and found abundant records for James (as mentioned), but never with the middle name, nor the correct parent's names and the dates did not seem accurate given what we thought he was most likely born in March 1864. I have found a Boston census record (via Family Search) which I believe is my grandfather and would put his birth year as 1865, again in March. On his Boston marriage certificate he gave his age as 29 in 1895 which would make his birth year 1866 so there are multiple conflicting dates (which is why I went through multiple years).
Thank you Elwyn especially for the details on the counties--I was searching GRONI in the Fermanagh and Enniskillen files but will go in again for Irvinestown and Ballyshannon just in case.
Yes, James emigrated to Boston in 1880. He married in 1895. Died in 1923.
Regarding Carolyn's note about the age--my father was in his 50's when I was born (I'm the youngest of our family); my mother was 14 years younger than my father and my father has been deceased for over 40 years (both my parents were deceased when I was in my early 20's before I thought too much about family history) so yes, James Bernard, born in Garrison sometime around 1864 is my paternal grandfather.
And yes, my brother did receive his Irish passport quite some time ago (confirmed) and probably when less paperwork was required. But he did talk about finding the records when he was there (in Garrison) and as noted, it could have been a baptismal certificate. My brother didn't decide to stop seeing/speaking with his siblings until some years later but when my sister wrote to him about the details he wouldn't engage.
I did look at Roots Ireland but recall not having much luck and didn't concentrate there.
Great grandparents: John Maguire & Margaret Cullen (do not have middle names); and the records on their location is unclear. The info we have says County Fermanagh but the town name seems to be a mistake or misspelled--it says Bumoran and I've found nothing that matches. I know there is a Bundoran in Donegal.
James Bernard did have siblings (I don't have details on years, except for a best guess on Patrick): Hugh Maguire who remained single/in Ireland; Mary Maguire who also remained single/in Ireland; Patrick Maguire (estimated birth year 1874, as he was known to join James in Boston for his brother's wedding (8/1895 when he was 21), Patrick married in Boston in 1908 and remained in Boston. Additional James siblings; John Maguire (married in US but we don't have details or family name of wife); and Bernard who married Catherine Quinn in Ireland, and remained in Garrison (they are the parents of the cousins my American cousin met in 2004).
Thank you all again, I'll dig back in with the additional locations and see if I have luck. The church records in particular seem to be much harder to decipher.