In my opinion, fir'Bolg (and bre'Unor) are simply a tribal nation type of thing - another group of humans. I see no evidence to disregard this, except for Connacht being Emperor of all human lands - though bare in mind that in those Dark Ages they would have had little knowledge of the outside world (so in Connacht's time they may not even have known of the Ermine's existance and it's peoples), and also, in Mediaeval societies people are much more egocentric are mistrusting of foreigners - when they said of all human lands in Connacht's time, they may have only been talking about those humans they viewed as important (those who fought against Moagim and the Myrkridia), and may even have viewed the fir'Bolg as sub-human (like in Tolkien's works, where the Elves don't recognise the forest-dwelling Petty Dwarves as properly sentient, and so hunt them as game, and then 6,000 years later when the forest-dwelling Drúadain aren't recongnised by the Rohirrim and Gondorians as properly human, and again, hunted.)
However, others here take them as being entirely different races.
