Irish surname evidence

Reilly surname origin and Irish county evidence

The strongest processed county signals for Reilly include Sligo, Waterford, Cavan. These are evidence clusters rather than proof that every Reilly family began in one place.

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Name history

What the historical record says

Reilly is the anglicised form of the Gaelic Ó Raghailligh (also written Ó Raghallaigh), meaning "descendant of Raghallach," a personal name borne by a tenth-century ancestor. The name was anglicised by dropping the Ó prefix from O'Reilly, yielding Reilly and related spellings such as Riley and Rielly. The sept rose to dominate the kingdom of East Breifne, or Breifne O'Reilly, corresponding broadly to County Cavan, where they ruled as lords from the thirteenth century. Their power extended into neighbouring parts of Meath and Westmeath until the Cromwellian confiscations broke their landholding.

Recorded Gaelic root: Ó Raghailligh (also Ó Raghallaigh)

County evidence

Strongest county signals

  1. Sligoprocessed surname/place signal
  2. Waterfordprocessed surname/place signal
  3. Cavanprocessed surname/place signal
  4. Fermanaghprocessed surname/place signal
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What this evidence can—and cannot—prove

Census clustering can show where a surname appeared strongly at a historical point in time. It cannot identify your exact ancestor, prove a clan relationship, or assign a coat of arms. Use it as a source-backed starting point alongside family records and parish research.