Mor O'Brien - Hmm...

It would've been helpful if FMG's 9 May 2022 update included William having married Mor O'Brien. Who knows if some future primary source will ever confirm it.

http://fmg.ac/Projects/MedLands/IRELAND.htm#_Toc389126191
D. LORDS of CONNAUGHT (de BURGH)
The primary sources which confirm the parentage and marriages of this family have not yet been identified, unless otherwise stated below.

WILLIAM de Burgh, son of --- & his wife Alice --- (-[1205/06]). His parentage is confirmed by the charter dated 7 Oct 1234 which records payment of a fine by his son "Richard de Burgh" for return of land in Connaught which had been confiscated from him "the strife with Hubert de Burgh Earl of Kent his uncle"[467]. Lord of Connaught.
m ---. The name of William´s wife is not known.

Ken