Machaire Gaileng, the plain of the Gailenga, remained under the control of the Gailenga until the 11th century,¹ when it was annexed by the O’Rourkes of Breifne. At that time, another Breifne tribe, Muintir Maelmórdha, included a sept that would later adopt the surname O’Raghallaigh (O’Reilly). The O’Reillys were subordinate to the O’Rourkes and, by the early 12th century, were installed as vassals to control Machaire Gaileng on behalf of the O’Rourkes.²

O’Reilly subjugation to the O’Rourkes led to hostile relations between the two families, When Tiernán O’Rourke, King of Breifne, was betrayed and killed by Hugh de Lacy in Athboy, the O’Reillys took the opportunity to break free from O’Rourke control. They allied with de Lacy, driving the O’Rourkes back into western Breifne and briefly seizing control of the entire kingdom.

The rivalry culminated in the Battle of Magh Slécht in 1256, where the O’Reillys faced a combined Connacht-O’Rourke army.³ The O’Reillys were renowned for their strong cavalry,⁴ and according to the annals, the East Breifne army nearly broke through enemy lines three times. However, they were ultimately defeated, losing their king, leading general, and several princes.^{5, 6} The O’Rourkes, although victorious, were unable to consolidate their advantage. They also suffered heavy casualties, and the following year, they themselves became embroiled in a war with Connacht. From this point onward, Breifne remained divided into East and West.

Relations between the Normans and O’Reillys were fractious: sometimes violent and other times peaceful. An example of this being that between the years 1380 and 1425 the Breifne men had destroyed 18 castles within the Pale,⁷ yet at the same time frequently intermarried with those Norman lords who settled along its borders.⁸ Furthermore, despite resistance to having Gaelic Irish in positions of religious power within the Pale, the O’Reillys wielded enough influence to hold the abbacy of the Augustinian Canons in Kells on numerous occasions during the 15th and 16th centuries.⁹

The O’Reillys were also renowned castle builders, with various princes constructing fortresses across East Breifne. Among these were the castles at Mullagh and Muff. Conor Mór O’Reilly built the castle at Mullagh, known as Cloghballybeg, on the shores of Mullagh Lake, while his son, Conor O’Reilly, constructed Muff Castle. The chief O’Reilly stronghold, however, was at Tullymongan in Cavan town, which became a significant market town strategically located between the Pale and the Gaelic north.

Despite becoming vassals of Tyrone in the early 15th century and later having their power curbed when East Breifne was shired into County Cavan, the O’Reillys retained control of the region until the Plantation of Ulster.

References

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1. Dáibhí Ó Cróinín, A New History of Ireland: Prehistoric and Early Ireland (Oxford, 2005), p. 884
2. Denis Casey, ‘A man of great power for a long time’ : Tigernán Ua Ruairc and the Book of Kells. History Ireland Magazine, Published in Features, Issue 5 (Sept/Oct 2010), Pre-Norman History, Vol. 18
3. The Annals of Connacht, Year 1256.5, Available at https://celt.ucc.ie (Jan. 31, 2025)
4. Philip O’Connell ‘Historical notices of Mullagh and district’ in Breifny Antiquarian and Historical Society Journal, Vol. 1, No. 2 (Cavan, 1921), p. 115
5. John O’Donovan, Annals of the Kingdom of Ireland by the four masters, from the earliest period to the year 1616, Vol. 3 (Dublin, 1851), p. 357
6. The Annals of Connacht, Year 1256.6~1256.7, Available at https://celt.ucc.ie (Jan. 31, 2025)
7. Geoffrey Keating, The History of Ireland from the Earliest Period to the English Invasion, trans. by John O’Mahony (New York, 1857), p. 732
8. D.B. Quinn and K.W. Nicholls, ‘Ireland in 1534’ in T. W. Moody, F. X. Martin and F.J. Byrne (eds) A New History of Ireland: Volume III: Early Modern Ireland 1534-1691, Vol. 3 (Oxford, 2009), p. 5
9. S. Booker, Cultural Exchange and Identity in Late Medieval Ireland (Cambridge, 2018), p.133

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