"AND I SAID LET.. GRIEF.... BE A FALLEN LEAF... AT THE DAWNING OF THE DAY"

So, finding himself at the dawn of day (the dawning of this relationship with Myriam), he compared grief to a fallen leaf. Now who worries about Autumn's fallen leaves at the onset of a pleasant summer's day?

This obviously refers to the fact that he consciously decided to act on his feelings for Myriam, despite realizing that he would ultimately be hurt.

At the dawning of his relationship with Myriam (the dawning of the day) he felt it convenient to class the unignorable "grief" that he knew would inevitably ensue, as the unavoidable remnants of the relationship that he nevertheless wanted to have(just as a fallen leaf is the unavoidable result of a pleasant summer's day)

Just like we feel that the very existence of a summer's day far outweighs the resulting fallen (dead) leaf, he similarly felt that the ensuing grief was not reason enough to decline the wonderful chance of happiness, albeit for a finite length of time.

Alternatively, he may have been saying: Ok, grief will ensue, but see it as a symbol of the joy it brought before it... like a fallen leaf.

In summary, he felt it covenient, while he was at the dawning of the day, to classify grief as something that would be found far later (at the closing of the day)... a time far enough away that he could ignore it for a while!

Indeed it is wise of Patrick Kavanagh to have stated that the sadness and grief that follows a break-up might better be viewed as existing only by virtue of the fact that extreme happiness and contentment preceded it.

I think though that Patrick Kavanagh recognised that his inevitable grief would be caused not only by the very fact that he was involved in a relationship with a girl but that he was involved in a relationship with this particular girl... a girl that Yeats might well have described as a "Maud Gonne"-ish type girl with "beauty like a tightened bow"

PS: I love Luke Kelly's version, raw and emtoive.