Ged has given a perfectly reasonable explanation for the counting chorus. I would go one even simpler and say it was just a clever little trick for showing off. Certainly none of this romantic mysticism involved (IMO). The same chorus is used in English songs in the 'No, Sir, No'/'Ripest Apples' family Roud 542. Jim Dixon gave a version here in 2010 & Karina in 2009. I have broadsides dating back to the middle of the 18th century. The Irish, American and Scottish versions seem to be developments of the English broadside.