This has really been bugging me. I have always pronounced Firbolg as Fur-bolg, but I have heard other people pronounce it like Fear-bolg. What is the official pronunciation?
Dndbeyond.com has a pronunciation button next to the main title of the Firbolg page that gives you an audio clip.
They pronounce it as fear-bolg.
You pronounce it feer-buhl-uh g
Firbolg is taken directly from the Irish (Gaelige) legends. It is the name of the race that, in Irish mythology, were defeated by the Tuatha DeDannan.
In Irish it's literal meaning is the "men of Builg", ie the men of the tribe of Builg or the men descended from Builg.
(Source: I'm Irish, and studied Irish mythology as part of my schooling as well as growing up in Ireland, I also consulted with some fluent Irish speakers I know [and live with])
In Irish Gaelic, "fir" (pronounced "fihr" to rhyme roughly with the German "ihr", but "feer" and "fear" in English aren't too far off, just a bit long with the vowel sound) is the plural of "fear" (pronounced not too dissimilarly to "far" in English), meaning man.
"Bolg", means "belly", "fat", etc. and is pronounced a bit like "bull-ug" in English, as if there were a schwa between the l and the g.
So, "fihr bull-ug" would be my closest approximation.
(14 years of Irish Gaelic in school!)