McShain is born Mary Horstmann in Philadelphia, Pennsylvania on March 27, 1907. Her parents are Ignatius J. Horstmann and his wife Pauline. She is the fifth of six children. She attends St. Leonard’s Academy in Philadelphia and Rosemont College in Rosemont, Pennsylvania. In 1927 she marries John McShain, building contractor who works on the reconstruction of the White House and the building of the Jefferson Memorial, the Pentagon, and the John F. Kennedy Center for the Performing Arts. They are both interested in horse breeding and racing, establishing a stable of racehorses in 1952. They expand this stable to Ireland in 1955, hiring first Vincent O’Brien and then John Oxx as trainers. Their greatest success is the horse Ballymoss.
The McShains move to Ireland in 1960, buying Killarney House in County Kerry and a large portion of the Kenmare estate, which had been owned by the Earls of Kenmare since the 16th century. They gift Innisfallen Island and the ruins of an abbey to the Irish state in 1973, bestowing guardianship of Ross Island and its castle to the state. For a nominal fee, they turn over the entire estate to the state in 1979, stipulating a life tenancy of the house and some land, with the rest of the land being incorporated into Killarney National Park.
McShain is a Dame of Malta and a Lady of the Grand Cross of the Holy Sepulchre. She receives the Pro Ecclesia et Pontifice cross in 1976. She is awarded two honorary doctorates in 1977, one from La Salle University in Philadelphia and one from her alma mater, Rosemont College.
McShain dies at Killarney House on December 2, 1998. She is buried beside her husband in Holy Cross Cemetery, Philadelphia. Her daughter, Pauline McShain, is a sister of the Society of the Holy Child Jesus and dies on March 8, 2019 due to complications of pneumonia.
(Pictured: John and Mary McShain with Lakes of Killarney in the background sometime in the late 1950s)