- Kevin Barry (briefly) – Irish republican (executed in 1920)
- Paul Dean – Former Irish and Lions rugby player
- Vincent Dowling – Irish American Director
- Frs. Tom and Earnest Farrell – Founders of the Catholic Boy Scouts of Ireland in 1927
- Larry Gogan – broadcaster
- Denis Hickie – Former Leinster, Irish and Lions rugby player
- Shane Jennings – Former Leinster and Irish rugby player
- Nicholas Kearns – Judge of the Irish Supreme Court
- Kieran Lewis – FormerMunster player
- Darragh MacAnthony – Chairman of Peterborough United FC
- Ronan McCormack – Former Leinster and UCD Rugby Player
- Jack McGrath – Leinster and Irish rugby player
- Rodney O'Donnell – Former Leinster, Irish and Lions rugby player
- Tom O'Higgins – former Chief Justice of Irish Supreme Court
- David O'Sullivan, Director General for EU Trade
- John Mark Redmond – cardiothoracic surgeon and businessperson
- Eoin Ryan – former MEP for Dublin
- Jonathan Sexton – Leinster, Irish and Lions rugby player [1]
- Tony Ward – Former Irish and Lions rugby player and League of Ireland footballer,
- Stephen Grissing - Former Leinster Rugby Player. Chief baller Davy [2]
Notable staff
- Éamon de Valera – Taoiseach and former President, taught at St. Mary's for a short time in 1915 as a mathematics professor