Here's the response I got from Tom & Barbara Brown about this song:

The song is properly called 'The Ox-driver's Song' - Cornish Ploughboys is really just a familiar title) and the bit that's probably unintelligible is the list of the oxen's names, so here goes:

For while still you are sleeping we rise in the morn
To plough for the farmer, so he may grow corn.
With Beauty, Spark, Berry, Gowdlock, Speedwell, Cherry,
Come whoop along, jip along, hark to us now,
For we are the lads that can drive on the plough.

Just in case they don't grow in America, the Gowdlock is the West Country name for the 'Gowan' (as they call it in Scotland) otherwise known as the Ox-eye daisy, and the Speedwell is a small wild plant with an intense blue flower - but here they're simply the names of the three pairs of oxen. Six oxen - big field - so I suspect the song doesn't originate in Cornwall as the fields are too small and they don't grow very much corn there anyway. There are a couple of other versions which have 'Hop along, jump along, here drives my lad along' instead of 'Whoop' and 'Jip' but I was told by an old guy who used to work horses that 'Whoop' and 'Jip' are the correct calls used to turn the team left or right respectively at the end of a furrow!