For anybody who wants to get out an instrument and actually play the tune, you have to go down to the bottom of the page I linked to see the notation.

There are a number of definitions of 'modal.' I go with the one I learned at an early-music workshop. In a mode, one takes a different note of the scale and treats it the way we usually treat the tonic - the tune starts and stops on it, and it may be important in some way elsewhere. In this song, the second note of the scale is the highest note in the song.

A tune which treats the second note of the scale as the tonic is Dorian. That's what this tune is doing.

Another Dorian tune I know is 'Savior of the Nations, Come,' which was published in 1524. I've heard rumors that this is an old chant tune.