Mac Diarmida was bred by John Hartigan at his Cashel Stud farm in Ocala, Florida.
Sired by Minnesota Mac, he was out of the mare Flying Tammie, a daughter of the 1958 Kentucky Derby and Preakness Stakes winner, Tim Tam.
As a weanling, John Hartigan sold Mac Diarmida in a private transaction to Dr. Jerome Torsney, who entrusted his race conditioning to future U.S.
However, at age three he had a Championship year, winning twelve of his fourteen starts including ten in a row on grass surfaces.
Ridden by Jean Cruguet, among Mac Diarmida's 1978 wins, he beat older horses in capturing the two most important North American turf races with European competitors: the Canadian International Championship at Woodbine Racetrack in Toronto, Ontario, Canada and the Washington, D.C. International at Laurel Park Racecourse in Laurel, Maryland.