The history is a layered thing. The present Sligo Grammar incorporates part of the Charter School set up by Royal Charter in 1752 and closed in 1843. It also incorporates the Diocesan School in Elphin, which moved to the Charter School buildings in 1862, bringing with it the ghost of its most famous pupil, Goldsmith. In 1907 the Incorporated Society closed its boarding school at Primrose Grange under the slopes of Knocknarea and built dormitories and classrooms on the current Mall site. In 1947, Sligo Grammar absorbed Sligo High School, founded in 1911 by Dean Ardill. The Hermitage - a residence beside the Grammar - was bought from the Campbell family to house the girl boarders. The Hermitage burned down in November 1976 and was rebuilt by September 1978. The school is what it is because of what it has annexed.

The roll of old pupils punches above the school's weight. Oliver Goldsmith (1728-1774) studied at the Elphin school that would later merge with the Grammar; he went on to write some of the most enduring poetry and drama of his century. Jack B. Yeats (1871-1957), brother of W.B., trained the painterly eye that would make him perhaps the foremost Irish artist of his generation. R. M. Smyllie (1893-1954) edited The Irish Times during the difficult years of the Second World War and after, presiding over a newspaper navigating censorship and political pressure with a wit and a pipe famously equal to the task. Frank Wynne (b. 1962) is one of the most awarded literary translators working in English today. Cathal Sheridan plays rugby for Munster. The list reads less like a school catalogue than like a small Who's Who of Irish cultural life.

The school sits on The Mall in Sligo, north of the Garavogue river, in a cluster of Victorian buildings expanded in 1971, 1985, 2002, and most recently in 2012 when a new library and classroom block opened. It is co-educational, fee-paying, and run under Church of Ireland management - one of the small group of Protestant boarding schools that survived the demographic and political pressures of twentieth-century Ireland. Approximately 450 students attend, about 100 of them boarders. Beyond the rugby - which is where the school is best known nationally - there are hockey teams, debating teams in English, Irish, and German, and an SATB choir that still meets each week to sing in four parts in a town first famous for its abundance of shells.