In the book 'Irish Ballads' (1996), Benedict Kiely, then working with Kavanagh on the 'Standard' newspaper recalled,                                                                                       
'As we sat talking in that newspaper office, [in] came Patrick Kavanagh with a few sheets of paper in his hand…could we sing that ["On Raglan Road"] he said, to the tune of the "Dawning of the Day"'.

So Kavanagh himself set the melody, not Luke Kelly, or anybody else.

The poem was indeed originally called 'Dark Haired Miriam Ran Away' and Miriam was indeed a girlfriend of his brother Peter.

The poem first appeared in the 'Irish Press' newspaper on 3 October 1946 (tho I don't know which title was used on that occasion).