Kavanagh would certainly have known Fáinne Gheal an Lae - it was sung much more then than it is now. But I don't know if he even intended Raglan Road to be sung to the same tune.

As for the use of the phrase "the dawning of the day", there are certain phrases that are stock in Irish ballads; this, I think is one of them. Another is Yeats's favourite "I shall arise and go now". Kind of like "I woke up this morning" in blues.