Among the almost 400 people present for the official opening were two past pupils who travelled from Florida, in the U.S.
They were Fr Michael Devine and Fr Aidan Hynes.
Fr Devine was a boarder from 1945 to 1950.
From 1960 to 1984, he taught English and Latin in the college.
Of his student years, he recalled: "I think the camaraderie that developed among the boarders was special.
"There were about 100 boarders and 60 day boys.
"The boarders were not allowed outside the school but the day boys brought us up things like chocolate."
Recalling heavy snowfall in 1947, Fr Devine said: "We wouldn't have survived only for the day boys who brought us food.
"Of course we paid them but there were food rations after the war.
"Some of the day boys' mothers took pity on us and sent us cakes."
When he returned as a teacher in 1960, Fr Charlie Kelleher was president of the college.
He was succeeded by Canon Tommy Finnegan.
Fr Devine recalled: "That was a time when we branched out to many things.
"I think I led the first Summerhill team to Liverpool to play a soccer game there.
"Afterward, we went to see a game in Anfield.
"In 1971, we were invited to take part in the European high school soccer championships in Bern, Switzerland.
"The tournament was over three days but we spent five days there."
"I hope it will repay the day boys in some way," he joked.
Looking back on his student days time in Summerhill, Fr Devine said many of the relationships formed continue to this day.
"In my time, there was John Mullaney, Joe Jennings and Paschal Morrison.
"We always get together in Dublin and have a Mass for the deceased members of the class. So the link continues."
It's the friendships Sligo town native Fr Hynes also remembers.
He was a student in the college from 1966 to 1971.
He said: "I have happy memories of Summerhill that have left me with an abiding affection for the place.
"The friendships I made were friendships for life."
Speaking about the new €13m school, he said: "I am so just happy for the present generation."