The song Master McGrath can also be found in volume 3 of Folksongs & Ballads popular in Ireland, ed. by John Loesberg, Ossian Publications. For what it is worth, here is what his notes say:

Master McGrath was named after an orphan boy who reared him in Colligan, Dungarvan and was two years old when he commenced running for his first Waterloo Cup in 1868. The dog had the distinction of being presented at court prior to the race and being petted by "all members of the royal family present". Lord Lurgan, its owner, stated that at the Waterloo Cup races of 1870 McGrath lost through foul play. The owners bookmaker had convinced the trainer to poison the poor dog causing it to loose that time. A memorial of Master may be seen just outside Dungarvan and strangely enough another may be found in Culford Hall, Bury St Edmunds, showing the respect he earned even with the English competitors. The tune is akin to: "Sweet Betsy from Pike" and "Still I love him".