Not to do with the Irish "Rocky Road" song, but "Rigs of Barley".
I've always understood the terms "Rigs of corn" and "Rigs of barley" as used in Scottish dialect to refer to the stacks of grain in the field that look like small tents or "ridges". "Corn" refered to oats and "baer" was barley. Since different crops were grown in the fields each year a "rig of corn" would be unlikely to be applied to an actual piece of land.
The method used to reap grain in Burns' time was to scythe the plant near the bottom of the stalk and gather the stalks together into sheaves ("thraves") tied with a couple of the stalks. Because of the general dampness in the Scottish climate, 8 to 10 sheaves were stacked together in a tent-like formation to be left to dry before "thrashing" the grains out.
Trying to make love in a field full of corn stubble was a bit chancy (even worse than on a "Rocky Road"), so a few sheaves could be laid down to make a bed upon which the young lovers could make merry. Hence the song's celebration of the "rigs".
This same method of harvesting grain on Scottish farms continued well into the 1970's (using tractor-drawn "binders" - reapers which cut the stalks and bundled them into sheaves tied with twine. "Binder Twine" was the farmers' answer to almost anything that needed repair - kinda like Duct Tape these days!) The stacking of the sheaves into "stooks" was still manual labor. As in Burns' time the field was ploughed over in the "Back-end" (Autumn) turning the stubble and roots of the plant back into the soil.
Modern combine harvesters have done away with the "Rigs of Barley".