Well there is the Welsh Hymn. Dod ar fy mhen dy sanctaidd law. We sang it like this in primary school:

X:1
M:3/4
L:1/4
K:Cmaj
G | GEG | FDF | EFE | D2
w:Dod ar_ fy mhen_ dy sa-nc-taidd law,
D | EFG | AFE | D2
w:O dy_ner Fab_ y Dyn;
G | AcA | GEG | AcA |G2
w:Mae ge_nnyt fen_dith i_ rai bach
G | AcA | GED | C2 |]
w:Fel yn_ dy oes_ dy hun.

but I have also known it sang to "Amazing Grace". I can't quite convince my self that the reverse (singing Amazing Grace to the above tune works well though)