Hi Mike!
Yes, WIlliam would appear to be the brick wall. Going from Elwyn's suggestion I actually worked with a very helpful researcher at PRONI, and they were unable to find any marriage records using (he looked at a 30 year range, I want to say) that matched the one at the Hants County website, which indeed comes from the Noel and the Genealogy of the O'Briens.
I have a copy of that I received from the Minnesota Historical Society's Library btw (very clean if you'd like a copy), as well as hundreds of pages of journals from Wetmore O'Brien (pretty sure his full name was John Wetmore O'Brien) that date from the late 1800s, the info in Wetmore's journals is likely quite a bit of the source material for the book. I am intrigued about the line in the book mentioning a fellow who visited Northern Ireland and visited the graves of his ancestors.
Have you done any YDNA testing? The Earl of Inchiquin connection is complete rubbish, I found the fellow who invented that profile and he took more than a few liberties. I have an entry for my father with William as the source here.
The O'Brien Project is here.
On the religion aspect, Ester's line can be traced back well over another century, she may have been born in Northern Ireland but her ancestry is decidedly Scottish. I find none of this surprising, heat maps of the M222 YDNA shows hits at a roughly even latitude going through Northern Ireland and Scotland. These folks all knew how to sail, after all. In the big picture it would seem to me that Hadrian's Wall marks the furthest north the Romans made a mark, so connections dating to that period across Scotland and Ireland would make plenty of sense. William or his ancestors may have converted earlier to save land, or William may have done it for love, we'll probably never know. I have been reading up on the history of Derry and Londonderry, and it also seems possible that William or his parents were some of the Gaelic Irish caught inside the walls (what today we would probably call the wrong place at the wrong time) during the siege or other military occupation/struggles and ended up forming relationships with the other urbanites there.
I wish I could find out more about the O'Braoin>O'Brien confusion. This is very intriguing but specifics of when the names were conflated would be required to be helpful for our purposes.
http://www.libraryireland.com/names/ob/o-braoin.php
Ó BRAOIN—I—O Brean, O Breen, O Bruen, O Browne, (O'Brien), Breen, Bruen, &c.; 'descendant of Braon' (sadness, sorrow); the name of several distinct families, of which the following are the most important: (1) Ó Braoin of Breaghmhaine. The head of this family, which is descended from Maine, son of Niall of the Nine Hostages, was lord of Breaghmhaine, now the barony of Brawney, in Co. Westmeath, adjoining Athlone and the Shannon. The family still flourishes in this ancient territory, but the name has been incorrectly anglicised O'Brien, which somewhat obscures its origin.
We're descended from Henry D O'Brien, (John) Wetmore's son, btw. He was awarded a Medal of Honor for his actions at Gettysburg with the First Minnesota (http://firstminnesotafilms.org/men/henry-d-obrien/). It's a pretty amazing story, and I'd hope our Northern Irish kin would be proud of him - it is apparently not at all a stretch to say that this unit singlehandedly swung Gettysburg, which is when the momentum turned to the Union.
Wetmore's line is: http://www.ancestry.com/genealogy/records/john-wetmore-o-brien_156147040, btw.
Very glad to meet you!
Carter