Legend has it that the Firbolg were enslaved by the Greeks and forced to move large volumes of soil in bags, which may account for the derivation of their name. For three centuries their persecution continued before they eventually stole some Greek ships and set sail for Ireland. The leaders of the escape were five brothers, Slainge, Rudraige, Genann, Gann, and Sengann.

The 5000-strong tribe headed to the west coast of Ireland but were soon scattered by the rough seas and had to land at different bays. They reformed at the Hill of Tara where the country was divided into five Provinces. These boundaries substantially survived into modern times and became four Provinces, with two of the original five being merged.

Ireland prospered under the Firbolg. They had a political structure, administration and a Kingdom. They brought bronze-age technology to Ireland.

Lady Wilde in 'Ancient Legends, Mystic Charms, and Superstitions of Ireland' describes:
'But the Firbolgs begin our authentic history. They had laws and social institutions, and established a monarchical government at the far-famed Hill of Tara, about which our early centres of civilization sprung, and where we have now most of those great pasture-lands, those plains of Meath that can beat the world for their fattening qualities, and which supply neighbouring countries with their most admired meats.'

They fought off persistent raids by the Fomorians, who they united with on several occasions to ward off other would-be invaders. For thirty-seven years there were seven successive Firbolg Kings who ruled over a thriving land in Ireland. But a new wave of invaders were on the way, the incredible Tuatha de Danann.

Despite negotiations and time-stalling tactics by the Firbolg, defeat to the technically superior Tuatha de Danann was inevitable. Staring defeat in the face the Firbolg petitioned the Tuatha de Danann for once last chance of victory: a battle between equal forces.

Bravery was not enough though. The Firbolg were finally defeated at the Battle of Moytura but not before they impressed the new rulers of Ireland with their fierce courage and honour. The country was divided again with the western part of the country, Connaught Province, being assigned to the Firbolg.

From this time on the power of the Firbolg waned. They continued to live in the West of Ireland and, together with the Tuatha de Danann and the Milesians, are regarded as one of the great ancient tribes of Ireland.