I'm not sure exactly, but I know that one of the lads in the Connells graduated school in the year74-75 and apparently thats got something to do with it?
The video makes it clear that the song is about the class of '74-'75 and I'm presuming all the people in the video are old classmates and not just actors.
But as to the lyrics .... they are a bit obscure. He sings about a YOU ... I'm presuming its an old high-school girlfriend- but I'll have to give it some more thought !