Good job on figuring out the name. It took me awhile to find a good recording of it, but I finally got it. This is an old recording from before 1989, but good enough quality.
The song is sang to a "mother", which is simultaneously Mother Bulgaria and a mother that has send her sons to battle. The lyrics have the repeated element of "oh, dear mother" over and over. Here is a rough translation of the rest:
Since the break of dawn,
an army is marching,
horse by the horse, hero by the hero,
horse by the horse, flag by the flag,
their lances are like a thick forest,
their swords are like a bright sun,
their clothes are like a dark cloud,
when they gallop, they are like the stormy winds,
they are lead by Tsar Ivan Shishman himself,
he leads them in the name of Bulgaria.
The song is old, nobody knows how old, and when things are passes by word of mouth they get often changed. Other versions of the song name the place of the battle (Sofia fields) and the name of the enemy (the Ottoman Sultan). There seems to be some disagreement among historians on which battle this song is about or even if such battle ever took place (the song sure is vague about details).
The "name of Bulgaria" might be a Communist change from "name of Christ".
I have seen even more versions of the song, listing the Tatars as the enemy, which would be totally ahistoric with both Ivan Shishman and Sofia. I have also seen a version describing an army with guns (bullets falling like hail). Those are obvious interpretations and modification.
Here is a more modern version:
http://www.youtube.com/watch?v=XjyUmXO8oCs&feature=related
(they sing well, but never mind the video)