Francis Patrick O’Brien gets a lot of selections in Gather 3rd ed. This is his Holy Thursday song and it goes forever (twelve verses) so you’ll get lots of foot washing done. Is it a lovely gentle tune.
There is a simple melody line for the refrain, so I assume that is for the assembly, but there is a harmony line to stop the monotony of thirteen repetitions. The verses have some differences in their tune and counter melodies, so it could be quite the production for a choir.
You can purchase the sheet music at GIA and they have the text there in the preview.
My version is just the main melodies.