Tom Kelly brews poteen "that exceeds them all." John's cow drinks up the still and wakes drunk with a broken horn. She makes a deal with Tom: if he won't tell John about her drinking she "will bring [him] against Lammas a fine heifer calf."
Morton-Maguire: John Maguire wrote the song on request of the schoolmaster who had kept John after school one day to inquire about John's cow and Tom Kelly's poteen. - BS