This being the eve of St. Patrick's day, it would be most appropriate to write about the Fir Bolg, a people described in the Book of Invasions as having been descended from an earlier people, the Nemedians, who were subjugated by the Fomorians. There are various theories regarding the derivation of the name, a possible translation of Fir Bolg would be "men with bags", according to legend, the name refers to a period of slavery, during which the Fir Bolg hauled dirt from fertile lowlands to less productive lands in leather bags. In some versions of the legend, these bags were sewn into leather boats, which were used by the Fir Bolg to sail to freedom in their ancestral homeland. Another possibly etymology would link the Fir Bolg to a Celtic thunder god, and attempts have been made to conflate them with the Belgae who Caesar mentioned in his Gallic War Commentaries.
According to legend, the Fir Bolg were defeated by the Tuatha Dé Danann in the First Battle of Mag Tuiredh, but were granted the province of Connacht due to their valor. In TheTain, Cuchulainn's foster-brother Ferdia was a noted Fir Bolg.
Despite there being no indication of superhuman stature in the Irish legends, the Firbolg (sic) managed to make it into the "Giants" section of the Monster Manual 2, along with the Fomorians of legend (another legendary foe of the Tuatha dé Danann). The Firbolg was presented as a sort of giant magical trickster, a leprechaun with a pituary condition. It was also the beginning of a wonky trend (continued into second edition AD&D) in which the damage inflicted by giants took the form (weapon type x N + strength bonus), rather than the classic Xd6 of the original Monster Manual giants. For game purposes, I like to conflate Fir Bolg and Gáe Bolg, so any Firbolgs encountered tend to be armed with double-sized spears and like weapons (tridents, partisans, spetums, and particularly harpoons- which seem to be a good substitute for the original barbed spear o' doom).
The computer real-time tactical game Myth: The Fallen Lords featured a really nice alternate portrayal of the Fir Bolg as forest-dwelling archers who, judging from the manual illustrations, came across as neo-Neanderthals.