My family is from
Ireland (I'm in the US) so this name doesn't seem strange to me at all. It reminds me of
Stephen Rea's character in "The Crying Game" before he changed it to
Patrick.
The only slight drawback with his brothers' names is you've already got an "F" name,
Finnegan. But with an "H" and an "L" name, it isn't so noticeable. Then again, if you call him
Gus (love it!) on a daily basis, then it isn't really an issue afterall. Go for it!
Fergus Oliver is terrific. Others you might like:
Fergus AlanFergus MalcolmFergus EdwardFergus JamesFergus NoelFergus CallumFergus DeclanFergus EamonnFergus Patrick (if you're a fan of "The Crying Game")
Fergus BenedictFergus Jude Fergus AubreyFergus KendallFergus KennethFergus KeithFergus RanaldFergus IvorFergus DanielFergus Martin