The Cistercian College in Roscrea, Co Offaly, has announced that it is to close after 112 years.
The all-boys boarding school at Mount St Joseph Abbey was founded in 1905 and currently has 167 pupils.
The fee-paying school will finish up the majority of its student services after the Junior and Leaving Certificate exams in June, the college said.
However, the school's management said it will consider continuing the Junior and Leaving Cert cycles for the current second-year and fifth-year pupils if there is sufficient demand.
The school, run by the Order of Cistercians of the Strict Observance (Trappists), said a significant fall off in student numbers is the main reason behind the announcement.
The fall off has been put down to a decline in demand for boarding facilities.
The monastery at Mount St Joseph Abbey is not affected.
"This is a very sad day for the staff, the students and their families, the many thousands of past pupils and of course the monks of Mount St Joseph Abbey. The decision to close was an extremely difficult one for the community to make," said Dom Richard Purcell, Abbot of Mount St Joseph Abbey.
"The school has witnessed a 45% drop in enrolment in the past 10 years with just nine first-year students enrolled for September 2017.
"Clearly this is unsustainable and the school is simply no longer financially viable. We were sadly left with no option but to conduct what we anticipate will be a phased closure of the school over the next 16 months," Dom Richard added.
"I wish to express my sincere gratitude to the Cistercian Monks (Trappists), and to the many other dedicated staff and students, who have carried out Trojan work in the school since its foundation in 1905," said Fintan Monahan, Bishop of Killaloe.
"The closure of Cistercian College will be deeply felt in our diocese," he added.
There are currently only five religious-run Catholic schools in Ireland that offer boarding facilities, compared to more than 30 such schools in 1990.
Cistercian College is currently just one of two remaining boys' boarding-only schools in the country.