Fir Bolg

Medieval Irish tradition presents the Fir Bolg as descendants of the Muintir Nemid (an earlier group who abandoned Ireland and went to different parts of Europe). Those who went to Greece became the Fir Bolg, who later returned to Ireland, settled and ruled the island for a time and divided it into provinces before being overthrown by the invading Tuatha Dé Danann (Wikipedia; Part 17 of The History of Ireland). In modern fantasy usage the name has been abstracted into a fictional humanoid race derived from that ethnonym (Wikidata; Pathfinder; Critical Role).

Medieval summaries cited here provide little physical description of the Fir Bolg as a people. In modern fantasy sources they are consistently depicted as large, tall, forest‑dwelling humanoids or giantkin: playable/encounterable tall forest humanoids in Dungeons & Dragons and Pathfinder, and described on Critical Role pages as among the tallest humanoid races (Critical Role; Pathfinder; Wikidata).

In the medieval narrative the Fir Bolg's notable actions are migratory and political — settling, ruling, and dividing the island, then being overthrown by the Tuatha Dé Danann; the provided medieval summaries do not ascribe specific magical powers. In modern game systems the Firbolg race is given mechanical traits that vary by system; for example, D&D rules discourse references a 'Firbolg Magic' trait allowing casting of detect magic and disguise self (RPG.StackExchange), and bestiary/character archives present them as playable/encounterable humanoids with system-specific abilities (Wikidata; Pathfinder; game archives).