This is a song in praise of the Virgin Mary (Marian Culture) which arose around the 13 century. There were many songs to do with this cult. This particular song is full of flattery to the Virgin Mary it appears to be in a south Midlands dialect.
The song comes from a manuscript found in the Llanthony Abbey Priory, the manuscript is now found in Corpus Christi College Oxford known as number 59.
There is an interesting page of information here
Here is the Melody,
Here are the words and a limited translation