Fir Ol nEchmacht was the name of a group or race of people living in pre-historic Ireland. The name may be translated as "men (Fir) of the race/people (Ol, possibly Dal?) of Echmacht" (Echmacht being the given name of the people).
Virtually all of Ireland west of the Shannon was once named after them, being called Cóiced Ol nEchmacht until the early historic era (c. 5th-7th centuries).
It was only with the rise of the Connachta dynasty that the term Fir Ol nEchmacht was dropped and the province was renamed Connacht.
References
Fir Ol nEchmacht Wikipedia(Text) CC BY-SA