Ó Siadhail / uaSiadhail / uaSiadgail is a Gaelic-Irish surname.
Those of Uí Failghe and Tír Chonaill were ollamhs of medicine, hereditary physicians to the ruling families in the respectives kingdoms and environs.
One of the Ó Siadhail branches were physicians in service to the McMahons of Oriel, and later to the O'Neills.
[2] The surname is now generally anglicised as O'Shiel, Shiel, Sheil, Sheils, Sheals, Sheal, and Sheilds (or Shiels, Shields as well as O’Shields), but the original form, Ó Siadhail, is used by persons who are Gaelic and conscious of their genealogy.
Notable people named Ó Siadhail or one of its variants include: