[Betis70]

I think it's originally Latin from the Requiem Mass (by Mozart). A way for Monty Python to make fun of Roman Catholics.

Pie Jesu Domine, dona eis requiem,
Blessed Lord Jesus, grant them rest,

Pie Jesu Domine, dona eis requiem,
Blessed Lord Jesus, grant them rest,

Pie Jesu Domine, dona eis requiem, sempiternam.
Blessed Lord Jesus, grant them eternal rest.

[Darksheare]

It was a pretty good bit in the movie though.
But the best part was the killer rabbit.