Sile Dennis, who helped organise Saturday's commemorative bash, told the Herald: "It was a really fantastic day and it was great for such a big star like Gabriel to come back and share his memories with everyone.
"Gabriel was a student at the school but he also taught history and Spanish here before he decided to become an actor. He was in great spirits as he caught up with many of his former pupils but also his former colleagues and classmates.
"He was really lovely and stayed for ages talking to everyone about his time in the school. He gave a really funny speech and staff presented him with a painting by Mo Kelly, whose grandfather founded the school."
During his speech to the 400-strong crowd, Gabriel fondly recalled his time working as a teacher in the school and revealed he only originally planned to leave for one year to try his hand at acting.
The Dublin-born actor, who claimed the school offered him a teaching job when no one else would hire him, also said he still has a clock given to him as a departure present and takes it with him everywhere.
The secondary school, which was founded by James J O'Byrne and his wife Esther, officially opened its doors to pupils for the first time on October 2, 1939.
The down-to-earth actor, who was recently named Ireland's new Ambassador for Culture, joined members of the O'Byrne family, pupils and staff to mark the special occasion.
Remembered as "a man ahead of his time", James O'Byrne had a deep belief in education for everyone and was one of the first to teach girls alongside boys at the voluntary lay Catholic Secondary School.
The historical school moved to Franshaw House in the early 1940s taking with it a teacher's desk which once belonged to Padraig Pearse.
There have been four principals in its 70-year history; James O'Byrne, Reiltin Ni Bhroin, Mattie Moloney and Etain O'Moore.
Today there are 80 pupils, 11 staff members and Eibhlin Ni Bhroin is the school manager.