Eoghan Ó Siadhail

Owen O'Shiel (; Ballysheil in County Down was the family's hereditary estate. where his son Owen O'Shiel was born in 1584. thumb O'Shiel then moved to Padua, whose university he considered the most prestigious and sophisticated in Europe. He remained there for an entire year, "observing the chief practitioners and anatomists [and] attending the lectures of the first chirurgeons, apothecaries and herbalists." O'Shiel's reputation grew and he received the degree of doctor.

He then spent half a year in Rome conversing with the best expositors of Galen and Hippocrates.

Early career In 1618, O'Shiel was recorded leaving Ireland for the Spanish Netherlands. O'Shiel received an annual pension from the Leinster nobility which would have allowed him financial independence.

Military career By 1631, O'Shiel had returned to the Spanish Netherlands and become the doctor to the regiment of Shane O'Neill, 3rd Earl of Tyrone, a position he held for at least two years. In 1638, he was doctor to the regiment of Owen Roe O'Neill. O'Shiel may have been present at the Siege of Arras in 1640. O'Shiel was well-known to military leaders Owen Roe O'Neill and Thomas Preston from his time in the Spanish Netherlands, and the two men solicited O'Shiel's assistance. O'Shiel decided he would return to Ireland. He was present at O'Neill's death outside Cloughoughter Castle in November 1649. According to historian J Duffy, Catherine inherited "the chivalrous fidelity of her father". O'Shiel and Catherine had multiple children, though their names have not been recorded. Owen O'Shiel was killed in the Battle of Scarrifholis near Letterkenny, where the Confederacy Army was defeated. He was found among the slain, heavily scarred and mutilated.

An eminent physician surnamed Shiel—possibly a descendant of Owen O'Shiel—resided in Ballyshannon in 1889.