Daffydd would tranliterate to something like the SE phonemes of DAH-vith. The double 'ff' makes the /f/ sound, whereas single 'f' = /v/ as there is no such letter in the Welsh alphabet. In Ar Hyd Y Nos I think I would render it something like Ar~rrrrr Heed Er Noss (like 'norse' without the rhotic sound!) So the two examples of 'y' seem to be pronounced very differently, depending on context.

(But bear in mind I am not even half-Welsh, indeed - just interested in language and living in Wales, so I am happy to be corrected by Auntie Myfanwy )