It is one of a category of hymns written in the 1960s / 1970s to teach people about what was happening at points within the liturgy. Over time the need for this genre has decreased, although these songs are still sometimes used in missionary countries, or when working with children.
Books it was published in include 20th Century Folk Hymnal Volume 1 and Hymns
Old and New for the Irish Church: it appears that it is not included in any currently in-print hymn books, and no on-line examples have been found.
With meter 22.214.171.124 it is set to an un-named tune which was originally published in the key of Bb.
LyricsThe words are copyright, but can currently be found here. There are four verses, each with four lines. A rough outline is:
1 The Mass is ended, all go in peace.
We must diminish..
2 We witness his love to everyone,
3 Go forth among the people, see men of every nation.
4 Thanks to the Father who shows the way,