The history of the present St. Mary’s has its roots in a pension school opened in 1887, in a prison block of the old Gaol bought by the Sisters of Mercy. Girls, ranging in age from 16 - 24, attended school in the early years.
In 1913 the ‘Round House’, or former Governor’s residence, became the Intermediate School.
1927 a Boarding School was opened in the building known as St. Catherine’s, the remodeled female infirmary of the gaol.
In 1929 the Intermediate School became St. Mary’s Secondary School.
Over the years it moved location more than once until in 1957 it finally moved to a new building – the core of the present school – in the convent grounds.
The centenary year was celebrated by building St. Joseph’s Block. In 1959-60, the boarding school was expanded and classrooms added. The boarding school was closed in 1972 and its facilities transformed into classrooms.
St. Mary’s was one of the first schools in Ireland to introduce Transition Year in 1975. A further extension was opened in 1980, providing office space and specialist rooms. In 1982 a special class for students with special needs was set up.
St. Mary’s completed the building of a new sports complex and classroom extensions in 2007.
St. Mary’s Secondary School is a Catholic, all girls’ school under the trusteeship of CEIST.