In Irish mythology the Fir Bolg (Fir Bholg, Firbolg) were one of the races that inhabited the island of Ireland prior to the arrival of the Tuatha Dé Danann.
In far antiquity the Fir Bolg were the rulers of Ireland (at the time called Ériu) immediately before the arrival of the Tuatha Dé Danann, or the Children of Danu, whom many interpret as the Gaelic gods. The King of the Tuatha Dé Danann, Nuada, sued for half the island for his people, but the Fir Bolg king refused. They met at the Pass of Balgatan, and the ensuing battle - the Battle of Mag Tuired - went on for four days. During the battle, Sreng, the champion of the Fir Bolg, challenged Nuada to single combat. With one sweep of his sword, Sreng cut off Nuada's right hand. However, the Fir Bolg were defeated and their king, Eochaidh, was slain by a goddess, The Morrígan, though the fierce efforts of their champion Sreng saved them from utter loss. The Tuatha Dé Danann were so touched by their nobility and spirit they gave them one quarter of the island as their own. They chose Connacht and are mentioned very little after this in the myths.
The origin of the Fir Bolg name is the subject of some dispute. Older commentators consider them the "men of (the god/dess) Bolg" or "men of bags" (compare Irish bolg meaning 'belly', 'bag').
These people arrived in Ireland in three groups, the Fir Bolg, the Fir Domnann and the Gaileanga. According to the model proposed by O'Rahilly: the Fir Bolg are linked to the historical Belgae, known from Gaul and Britain, and to the historical Builg of Munster; the Fir Domnann to the British Dumnonii; and the Gaileanga are the Laigin, who founded Leinster. According to this model, the three groups probably represent the Ivernic-speaking peoples who inhabited Ireland before the Goidelic-speaking Gaels.
Other theories have been advanced about the origin of the Fir Bolg. Some scholars have related the name of a Celtic god with the word Bolg. The Fir Bolg, according to one legend, were involved in carrying bags of earth at one point in their history, hence the "Men of Bags" interpretation. Others speculate that "Bolg" relates to a word for small boats.
One interpretation which has gained ground is drawn from the recorded histories. The Fir Bolg, according to this theory, were largely conquered by the Gaels, and thus, as a lower class in society, would have had different customs befitting a lower social status. In particular, this theory holds that "Fir Bolg" is a corruption of a term for "Breeches-Wearers", reasoning that, as manual labourers, the Fir Bolg would have found it useful to wear trousers rather than the robes and garb of the Gaels. This theory, however, remains largely speculative, and there is little hard evidence to confirm this interpretation.
The Fir Bolg were recorded as being ejected from Ireland and returning under a King named Aengus. The Fir Bolg were given, as a place of settlement, the Aran Islands and surrounding coastland (the largest of these Islands, Inishmore—Árainn—is home to a fortress allegedly related to Aengus and the Fir Bolg, Dún Aengus). This episode of history, in which the Fir Bolg come from what is assumed to be a place near modern Scotland, settle in Ireland, and then go to the Aran Islands, on Ireland's western fringe, has given rise to one interpretation of Fir Bolg origins. A Pictish invasion of Ireland is the proposition in this account, and the Aran Islands were a last refuge for this invading force.