A people who invaded and inhabited Ireland. They were defeated by the Tuatha Dé Danaan.
Origin of fir-bolg
Irish from Old Irish firplur. offermanwī-ro- in Indo-European roots bolgperhaps genitive plur. ofbolgbag (literally, “men of bags,” perhaps from the notion that warriors “swell” with heroic valor in battle) bhelgh- in Indo-European roots
American Heritage Dictionary of the English Language, 5th Edition