The Sisters of Mercy established the Secondary School in Trim in 1936. At that time, the school was attached to the Mercy Convent. The first school day was 14th September 1936, with an enrollment of 14 boys and 15 girls. The co-educational system only lasted one year. Mother Evangelist was the Principal from 1936-1949. The original teachers were Miss E Kelly and Miss M Burke. The school was quite small with 3 classrooms and a cookery room. The cost for one student was 2 guineas per term, rising to 10 guineas per year in 1966.

The Sisters of Mercy bought the current site in the late 1960s and built a new Secondary School that was officially opened in 1969.

The current site had been a jail from 1734 until closure in the 1880s. The jail then became a reform school for delinquent boys and there are many stories connected with this period. There is a poem entitled “The Trim Joint School (Life in an Industrial Boarding Hotel in 1900)” that describes life in the reform school for the boys. Today, the wall to the rear of the site is all that remains of the old jail.

On the official opening of the school in its current location, there was an enrollment of 240 students. This was an increase from 86 students in 1962-63 and 156 students in 1966-67. 1967 saw the introduction of free education leading to the large increase in enrollment. The Department of Education gave £15 capitation for each student. The total cost of building the school was £15,000.

There were 8 teachers on the opening day in 1969. They were Miss Malone, Miss McLoughlin, Miss K Kelly, Sr Anthony Murphy, Sr Brid Boland, Sr Mary Clavin and Sr Francis Looby. Sr Concepta was the Principal from 1949-1982. Sr Michael (Rosalie) Healy took over from 1982-1996. The first lay Principal was Jeremiah Kearney from 1996 until his retirement in 2007. The current Principal is Jacqueline Maher. Over the years, many extensions have been added to the original building of 1969.