Coláiste Muire was developed as a Girls Secondary School in 1942. Its beginnings were modest and the school operated from the Convent community room and a small room behind the convent.
In 1957, a new school premises was opened which had been entirely funded by the Mercy Order. This new school was one of the first co-educational voluntary secondary schools in the West of Ireland.
A fully equipped science laboratory was added in 1967. A further extension was added in 1979 providing additional classrooms and specialist rooms. It was around this time that woodwork and technical drawing were introduced.
A new extension opened in 1991 providing a second Science Laboratory, Library, Arts and Crafts Room, Career and Pastoral Rooms and offices. The 1991 extension also included a full sized Gymnasium equipped for a wide range of sports with a viewing balcony and adjoining changing rooms and showers, all opening out to a three acre playing field and two hard court areas.
Further classrooms were added over the next few years and in 2003 a new construction studies room was commissioned.
In 2008, the trusteeship of the Western Province Mercy Schools changed from religious trusteeship to CEIST (Catholic Education, an Irish Schools Trust).
Also in 2008, Coláiste Muire celebrated its golden 50th anniversary.