In Ireland, the McNulty family are a well known Donegal surname. The surname means literally 'Son of the Ulsterman' and in late medieval and early modern times the McNulty family were highly respected as hereditary doctors to the O'Donnell lords of Tír Chonaill. However, in earlier medieval times the McNultys, then known as the McDunlevy family, were actually kings of the Ulaid, the original Ulstermen, governing a kingdom which covered a vast area east of the River Bann and Lough Neagh, roughly the modern Irish counties of Antrim and Down. However, around the year 1200 AD the McNultys and the Ulaid were conquered by the Anglo-Norman conquistador baron John de Courcey. De Courcey established the earldom of Ulster on the ruins of the kingdom of the Ulaid and the McNulty family fled to west Ulster. Here the family took up the study of medicine and became hereditary doctors to the O'Donnells. The McNultys held this respected position until the collapse of the O'Donnell lordship which occurred from 1602-1610.
Had Jimmy McNulty been a real person his ancestors might have emigrated to Baltimore in the USA in the nineteenth century to escape poverty or perhaps even the Great Famine of the 1840s. Many Irish who emigrated to the east coast of the USA found employment in the police departments of the great American cities. Here they made a huge contribution to the social life of their adopted country. As such Jimmy McNulty is just the fictional representative of a real and highly valued aspect of Irish American life and culture.