I followed the descendants down to Thomas John Hamilton FitzMaurice (1803 - 1877) and could not find a problem nor a connection to you. You were not logged into wikitree so I could not see your connection to him through the connection finder. I guess we need a little more information.
That's a downstream match from Murrough O'Brien, the "William O'Brien KB, PC" listed is someone who was actually living in England. It's not the William O'Brien we descend from. Wrong birthdates in addition to different genetics - if you go to Family Tree DNA we are kit # 108748.
Looks like this one is the 98-year-old ancestor of the immigrant
But it's your source that says he descends from Brian Boru?
No, that link is indeed to "our" William O'Brien.
This is an issue I stumbled on years ago when a self-proclaimed "professional genealogist" living in Tennessee or something like that emailed me saying he could connect us (for a fee) all the way back to Brian Boru. What he had done was ignored birth date discrepancies (not to mention actual geography) and swapped in this "William O'Brien KB, PC" fellow for the William O'Brien you linked.
Looks like somebody else took the bait and paid the fee and the story is out there.
was originally created as the Derry boy and married to Esther, but the borrowed aristocratic parents were attached and the transmogrification continued from there.
Question of who the PM wants him to be.