Sometime round the 6th century Feargna migrated north, and was succeeded by a son Aedh Finn (Hugh the fair) who died in 611 A.D. Aedhs son Maelmorda was ancestor of the O'Reillys. This is why for much of the time they ruled what is now county Cavan, it was know as Muinter Maelmordha (The country of Maelmordhas people).From Maelmordha through Dubhcren and Cathalan came Raghallach, who was reputedly killed at the Battle of Clontarf. From him all the O'Reillys are named. Such is the ancient genealogy of the O’Reillys.
When the O'Reillys first appear in the Irish Annals they ruled a small area north of Lough Ramor, but shortly after they were crushed by the growing power of their western cousins the O'Rourkes who ruled what is now Co Leitrim. In 1161 Godfraid, great grandson of Raghallach, was killed in a battle near Kells, Co Meath, by Melaghin O'Rourke. The O'Rourkes under a strong King, Tiernan, were expanding into Meath. The situation was transformed with the arrival in Ireland of the Normans in 1169, the O'Reillys were one of the few Gaelic families to stand with the invaders during the critical early years, but then they had no stake in the old order. By 1172 Tiernan O'Rourke was dead killed at a parley by Hugh de Lacy, and the O'Reillys again gained a degree of independence.
This cosy relationship with the new comers seems to have continued until the early 1200s. In 1220 Hugh De Lacy invaded central Brefnie and captured the O'Reillys crannog at Lough Oughter. By 1224 the O'Reillys were besieging Lough Oughter, in 1226 they dismantled the Norman motte at Kilmore. In 1233 a De Lacy invasion of Cavan was repulsed with heavy causalites, at a battle in the Bellavalley Gap. It was the last serious challange to O'Reilly power until the Tudors came to power in England. The O'Reillys suffered a serious defeat at the hand of the O'Rourkes, and the O'Connors at Maigh Sleacht in 1256, in which Cathal, the Chief and many of his sons and brothers were killed. The battle was lost but they won the war and this was the last serious attempt by the O'Rourkes to dominate East Brefnie.
The power of the O'Reillys was secured by Giolla Iosa Rua, who became chief in 1293. In 1300 he granted land for an abbey at Cavan. His chieftancy lasted till 1330, and his sons extended their power by raiding widely into Meath. Thomas, grandson of Giolla Iosa, pushed O'Reilly power into modern county Meath, as far as Fore and the Lough Crew hills. He also built Crover castle to hold the new land.
In the late 1300s the O'Reillys shifted their seat of from Clogh Oughter castle to Tullymongan, on the hill above Cavan town. O'Reilly power was at it's height. They are an unusual Gaelic ruling family in many ways. Cavan is an medieval Gaelic town. It was a thriving market center, and the O'Reillys even minted their own money. They were also known for the quality of their horses, and the ability of their cavalry. But the annals are full of accounts of war, raiding, and murder. The O'Reilly occupied the border between two worlds, the Gaelic and the Norman. They were able to exploit the advantages that living on the border provided, to trade as well as to raid.
In the 1500s the Tudor state was exerting it's power all over Ireland and a border region like Brefnie was among the first to feel the pressure. The Anglo Norman Nugents and Plunketts from Meath, began to assert their power in Brefnie, on behalf of the English Crown. In 1553 Sir Thomas Cusack, Lord Chancellor of Ireland wrote "next to Annalie (Longford) is a large country called the Brenny wherin O'Rail is chief captain who has seven sons. He may make four hundred horseman of the same name and one thousand Kern (irregular soldiers), and two hundred Gallowglass (mercenary soldiers). The county is divided between them, which joinet to the English pale and upon a country called Plounketts country, betwixt which has been divers, murders, stealths, and robberies, by day and night committed".
As the century progressed the pressure became greater, in 1566 they signed the humiliating treaty of Lough Sheelin with the Earl of Sussex. In 1584 Brefnie was shired and became the county of Cavan. O'Reilly power finally collapsed in the wake of the Nine Year War (1594-1603). In 1600 Lord Mountjoy took Cavan town and placed a garrison there. In 1601 Edmund O'Reilly (of Kilnacrott) was killed in Cavan, he was the last of the family to hold the title "The O'Ragahallie", there were many claimants later, but none would ever be Chief of Brefnie.
Some members of the family did receive substantial grants of land in the Plantation of Ulster (1611), but dispossession in subsequent confiscation’s, an inability to balance the books, in the market economy that followed Plantation, and many members of the family choosing emigration to Catholic countries in continental Europe, resulted in the social decline of the O'Reillys in Ireland.
One branch of the family that did hold for longer was the Baltrasna O'Reillys. They were descended from Edmund of Kilnacrott. Myles O'Reilly is a semi mythical figure, a cavalry officer in the 12 year war, he was known as Myles the Slasher. His son John fought with the Jacobites in the Williamite war (1689-91), and only saved his land because he was specifically mentioned in military articles of the Treaty of Limerick. Johns grandson Alexander, had a distinguished military career in the Austrian and Spanish army, and giving his name to O'Reilly St in Havana, Cuba.