What happened to the Fir Bolg after Ireland was conquered by the Tuatha Dé Danann?
In Irish mythology, Ireland was once inhabited by the Fir Bolg, who were descended from the earlier Muintir Nemid. The Fir Bolg were later defeated by another group of Nemed's descendants known as the Tuatha Dé Danann in the Battle of Cath Maige Tuired. However, the victors gave the Fir Bolg a quarter of Ireland to live in. According to Wikipedia,
[T]he Tuath Dé offer them one quarter of Ireland as their own; they choose Connacht and are mentioned very little after this in the myths.
The races that occupied the land when the so-called Milesians came, chiefly the Firbolg and the Tuatha De Danann, were certainly not exterminated by the conquering Milesians. Those two peoples formed the basis of the future population, which was dominated and guided, and had its characteristics moulded, by the far less numerous but more powerful Milesian aristocracy and soldiery.
So what happened to the Fir Bolg ultimately? When were they last mentioned as a distinct identity?
According to a poem by MacLeigh which can be found and discussed in The Annuals of the Four Masters
They fled to Connaught and having seized power from the Milesians in the 1st century ruled there until the 3rd century, when Aodh son of Garadh King of Connaught and last king of the Fir Bolg was defeated by Cormac king of Ireland. The Fir Bolg never had any power in Ireland following this and were reduced to peasants of which a great many are still (as of 1499) considered to be of Fir Bolg origin.
From The Annals of Ireland Translated from the Original Irish of the Four Masters by Owen Connellan - Reign of Henry VII
So it would seem they became the lower rungs of society and could assume they are eventually assimilated into "the Irish people" through marriage etc.