Seems lots song writers [poets] maybe unable to compose original tunes for their compositions, or 3rd parties put poems & existing tunes together to create songs.

Some examples which spring to mind are:

On Raglan Road, orig a poem from Patrick Kavanagh which was given the melody of Fáinne Geal an Lae, a much older Irish air by Luke Kelly.

My Grandfather's Ferret to Henry Work's 'Grandfather's Clock'


Maries Wedding seems derived from Mo Ghile Mear {sp?}another trad Irish air tune.

Some've Percy French's output, such as 'Pride of Petravore' [Rights of Man]


Others?