do we know what happens to the fir bolg after they are defeated and given connacht? are they related to the connachtmen we see in the ulster cycle and later, or have they just faded into obscurity?
Ah, an excellent easy answer! The Fir Bolg are absolutely related to the Connachta of the Ulster Cycle. If you remember the Gailióin from Táin Bó Cúailnge, that's just the name of one of the three population groups that make up the Fir Bolg.
While from a modern perspective, where as a result of Tolkein's influence on the modern fantasy genre and how that influences how people approach early traditions, we often have a reflex to get very 'Taxonomic' when discussing things like Elves, Dwarfs, Fauns, and the Fir Bolg, that is a very modern view. The Fir Bolg are just some of the ur-peoples of various bits of Ireland (peoples move around, occasionally in strange and confusing ways), but especially Connachta.
While I highly doubt anyone in modern Ireland bases their identity around being a descendant of the Fir Bolg, they're just part of the genealogical tradition. Just a deeply mundane fact of the genealogies.
So, the Fir Bolg never faded into obscurity. They've always just been there, and technically still do if you take medieval genealogies seriously (again, let me stress, I am not saying that any people in modern Ireland go around claiming Fir Bolg ancestry, it is just that the Fir Bolg were just 'some dudes' in the genealogical tradition, and some people are descended from them like how the genealogies have everyone going back to Adam because, you know, he's Adam). They're not like a 'Species' or anything, just an ancient, imagined political faction.
Of course, if @margridarnauds has anything to add to that, I'd absolutely welcome it. I'd normally direct all Fir Bolg stuff over there, but since this seems like a rather simple one, I thought I'd avoid just throwing this off onto someone else's lap.
