The Fir Bolg (pronounced "fear bollug") were a nomadic race of near-giants that lived in ancient Ireland. Sometimes spelled as one word (Firbolg), the name means "bag men", an appellation generally attributed to the backpack-like bolgs which they wore around their necks.
The origins of the Fir Bolg are somewhat disputed by folk historians. They are generally accepted to be descendants of Neimheadh (sometimes written Nemed), who arrived in Ireland about thirty years after the death of the Partholonians by plague. Some accounts state that the bulk of Neimheadh's people fled Ireland after a fearsome battle with the Fomorians, but his grandson Semeon remained with a handful of people. These few made peace with the Fomorians and stayed in Ireland to prosper and multiply for quite some time.
Another account by medieval historians maintains that Semeon and his followers left Ireland and went to Greece, where they bred prolifically until they numbered in the thousands. They were then enslaved by the Greeks, who employed their great size and strength to carry loads of clay into rocky areas in an attempt to turn these barren lands into fruitful plains. They carried this clay in large bags worn about their necks, from which their famous bolgs and their name would later evolve. These laborers had three designations: the Fir Bolg, the Gaileoin and the Fir Domhnann. The Gaileoin were named for the "gai leoin", or "javelins of wounding" which they used to dig the clay. The Fir Bolg were named for the bags in which they transported the clay, and the Fir Domhnann were named from the deepness (domhaine) of the clay when they laid it atop the rocks. When they left Greece some time later and returned to Ireland, they left their roles of digging and laying clay behind, but their continued use of neck-bags perpetuated the name Fir Bolg, which became a general name for all of their race.
Both of these stories agree on what happened after this. The Fir Bolg, being strong and numerous, stood atop the hill of Uisneach (now Ushnagh in County Westmeath) and divided the land into its five provinces. The word "cùige", meaning "fifth", is still used to refer to Ireland's provinces, even though only four remain today. The Fir Bolg remained the sole inhabitants of Ireland for two more generations until the arrival of the Tuatha de Danaan. They arrived in a week of mist and fog, and asked that the Fir Bolg give half of Ireland to them to settle. The Fir Bolg refused, and the famous First Battle of Magh Tuireadh (Moytirra) ensued, in which the Fir Bolg were defeated.
Again, accounts differ on the fate of the Fir Bolg at this time, some claiming that they fled Ireland and settled in islands off the Scottish coast, only to be driven back to Ireland later by the Picts. Others maintain that the Fir Bolg remained in Ireland after the battle, acknowledging the superiority of the Tuatha de Danaan, but maintaining their own nomadic independence rather than becoming subservient. When the Milesians arrived and drove the Tuatha de Danaan to the Otherworld, the Fir Bolg joined the new society, albeit at the lowest rung of the ladder, giving their service as warriors to the high king of Tara, Cairbre Nia Fear (not to be confused with Cairbre Lifeachair, son of Cormac mac Airt). They later rebelled against him, only to have their four greatest leaders slain by four heroes of the Red Branch, including Cu Chulainn.
Despite this, the Fir Bolg were eventually accepted into society, although their status remained very low until the formation of the Fianna and the ascension of the charismatic hero Finn mac Cumhail as their leader. During his tenure as Rigfennid, leader of the Fianna, Finn brought previously unknown prestige to the warrior class. His warriors were elevated in status to the point where they married the daughters of craftsmen and artisans, and he himself married two of the high king's daughters. Late in Finn's life, a series of misfortunes combined with infighting among two of the major Fir Bolg clans and the horribly bloody battle of Gabhra, decimated the Fianna and the Fir Bolg race. The few scattered Fir Bolg that remained were ultimately assimilated into the "mainstream" Irish race and thus faded from history.
Footnote: To give credit where credit is due, the etymology of the three divisions of the Fir Bolg and their various roles as Greek slaves was taken from "Myth, Legend and Romance: An Encyclopaedia of the Irish Folk Tradition" by Dr. Daithi O hOgain. This encyclopaedia is an excellent reference work, if perhaps overly intellectual, and the only source I have found with such detailed information. The rest of the information in this write-up was gathered from a wide variety of sources on Celtic and Irish folk history.