County Cavan is central to the medieval O'Reilly story. The Irish name Ó Raghallaigh is recorded for rulers of Breifne and, later, East Breifne. Their political history unfolded across the Cavan–Leitrim region and remains an important part of Cavan's heritage.
For the name's meaning, Reilly spelling and wider Irish history, read the O'Reilly and Reilly surname history. This page follows the East Breifne story and the Cavan records useful to family researchers.
Cathal Ó Ragallaig and medieval Breifne
The Dictionary of Irish Biography identifies Cathal Ó Ragallaig (c. 1195–1256) as king of Breifne. With his brother Cú Chonnacht, he developed the dynasty into a major power in the Cavan–Leitrim region during the thirteenth century. This was a landscape of competing lordships and shifting alliances, not simply a modern county with fixed borders.
A later ruler, Matthew Ruad Ó Ragallaig (d. 1282), was king of East Breifne. The DIB account places him among the next generation of rulers whose fortunes shaped the region. These biographies provide named people and dates for the medieval chapter of O'Reilly history.
Where to begin a Cavan family search
The strongest bridge from a modern O'Reilly family to Cavan is a documented place: a parish, townland or civil registration district. Start with the most recent confirmed relatives and compare both O'Reilly and Reilly in each generation. Note neighbors, witnesses and occupations when several households share the name.
- Use National Archives of Ireland census records to find 1901 and 1911 households and townlands.
- Search births, marriages and deaths through IrishGenealogy.ie; connect each record with parents, spouses and addresses.
- Once a parish is known, consult church registers, land valuations and local collections to follow earlier generations.
- For families who emigrated, compare destination records with the Irish place recorded at home.