Sorry, a typo: it should be "buachailleacht bó" not "buaichailleacht bó" (many thanks to the alert readers who PM'd me on this!).

I believe that the song "Buachaill Ón Éirne" dates from the early 19th century in Ireland, whereas I recall reading that the term "cow boy" can be traced back to early 17th century England. I don't have an etymological dictionary at hand to check this.

Don