Belfast Telegraph columnist McCann, who went on to become one of the most prominent civil rights activists in Ireland, reminded Heaney of his 'digging' in later life - and joked with Ireland's most famous poet that he still owed him "a thump in the kisser".
McCann tells the story of his close encounter with Heaney on Eamonn Mallie Meets..., the new interview series hosted by the veteran political broadcaster.
"Heaney once did hit me, and he hit me from behind, which you're not supposed to do," McCann tells Mallie.
"Heaney was the head prefect you know, he was a good boy, and I remember he used to sit in the senior study where we used to go on free periods to study and do homework or whatever it was.
"I was chatting, sort of muttering and all the rest of it in the study and in classes and so forth, and suddenly I turned around and I heard this step and before I could turn around: 'Go! - it was Seamus Heaney hitting me."
But there were no hard feelings, McCann tells the programme, after he recounted the painful experience to Heaney much later in life.
"I reminded him of this at a wonderful event at the shores of Lough Graney in east Clare. He came over and recognised me and I immediately said to him: 'I owe you a thump in the kisser, Heaney'. And he remembered!"
Discussing national identity, McCann also reveals that he is a firm supporter of the Northern Ireland football team and would hate to see a situation where Protestants support one team and Catholics another.
Eamonn Mallie Meets... is broadcast on the Irish TV Channel on Sunday at 10pm on Sky or Freeview channel 191, Freesat channel 400
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