The High School was founded in 1870 by the Governors of the Schools Founded by Erasmus Smith, Esquire (commonly known as The Erasmus Smith Trust) which was established by Royal Charter in 1669.  The Erasmus Smith Trust is the ultimate authority in the conduct and ownership of The High and Diocesan School CLG.  The latter is controlled by the Board of Governors with oversight provided by a Board of Management on which parents and staff are fully represented.  The school is directed by the Principal supported by a staff of approximately 60 qualified, dedicated, and committed teachers with a wide range of skills and interests.

In 1971 the school moved from Harcourt Street to its present 23-acre site at Zion Road bringing with it over 100 years of distinguished educational tradition.  The High School became fully co-educational in 1974 following the incorporation of The Diocesan Secondary School for Girls which was founded in 1849.  This important development allowed the school to consolidate its existing links with Church of Ireland and other Protestant communities both within the city of Dublin and across the United Dioceses of Dublin and Glendalough.  A major new building programme was completed in 2001.  The High School will celebrate its 150th anniversary in 2020-21.