Mark Eden had his first starring role in this film, but during the third day of shooting he had to jump off a horse and run down a hill; he caught his foot in a rabbit hole and broke his ankle. This resulted in him being dropped from the film and losing his five-year contract with Disney.
"Red Hugh" O'Donnell (in Irish, Aodh Rua Ó Domhnaill) was born in 1572, a son of Hugh Roe (Aodh mac Maghnusa Ó Domhnaill), king of Tir Connaill. The movie, though heavily fictionalized, contains many elements from his life: He was imprisoned in Dublin Castle, but at age 15, not as an adult. He did escape twice, in 1591 and January of 1592, with the assistance of his friends the O'Neills--in fact, he and two O'Neill brothers are the only men to escape (during which Hugh lost two toes to frostbite). At 20 he became The O'Donnell, Lord of Tyrconnel (now Donegal, hence the movie's title), but on his father's abdication, not his death. In 1593 he declared war against the English, joined in 1596 by the O'Neills, though Donegal Castle was never taken by the English. After his forces won an impressive string of battles, Spain entered the conflict as Ireland's allies in 1601, but the combined force was defeated in 1602 and Hugh fled to Spain, hoping Philip III would provide more aid. Unfortunately, the Prince of Donegal died that year at Simancas Castle.