Coláiste an Chreagáin in Mountbellew is to stop taking in new students from September.
School management says first year students and transition years will not be taken in this year as the school moves towards becoming a further education and training hub.
The board of management met this week to discuss the challenges posed by declining enrolments.
From September this year, the school will accommodate less than 50 students in total.
Coláiste an Chreagáin says it'll continue to support current students to complete their studies, and a parents' meeting will be called shortly.
The school, previously St Jarlath's Vocational School, was established in 1932.
The board of management says the school will now adapt to the needs of the community by expanding its further education and training offerings.
The expansion in this sector will be supported by the Galway Roscommon Education and Training Board, and SOLAS.
