[8] During Marshal's punitive war in Ireland, chief Cathal O'Reilly, great-grandson of Geofraidh, exploited the power vacuum left by de Lacy to secure control of Breifne from his enemies.

For his assistance in the campaign against de Lacy, Henry III issued a royal grant to the King of Connacht giving him overlordship of Breifne, a position which he delegated to his son Aedh.

In 1233 William Gorm de Lacy, having received a royal pardon for his overseas service, attempted to retake Breifne with a large force of English and Anglo-Irish soldiers.

In 1256 Walter de Burgh raided into Connacht to devastate the country and relieve pressure on the O'Reillys who further north had ventured into western Breifne to retake control of the territory.

14th century East Breifne also saw the arrival of prominent Anglo-Norman families such as Lynch and Fitzsimons, as well as the Hiberno-Scottish mercenary clan MacCabe, who became gallowglass warriors for the Maguires, MacMahons and O'Reillys.

[23] The English invaded the territory in an attempt to install Eoghan na Feosaige as king but were repelled, as were the O’Rourkes of West Breifne who simultaneously attacked the east in order to capitalize on the situation.

[27] Due to East Breifne's location, the O’Reillys historically held a unique position as mediators between the English and Anglo-Irish of Leinster and the Gaelic lords of Ulster.

With the ever-present threat of further English expansion and its western rivals persisting with their centuries-old claim over the kingdom, political stability and the ability to balance strong alliances and play both sides were vital for the country's survival.

In 1533 King Fearghal mac Seaain appealed directly to the government in England for redress against Tyrone, bypassing the Fitzgerald viceroy, who raided East Breifne in response to this circumvention of their authority.

O’Reilly continued to serve the crown's interests, arranging a meeting between Lord Deputy Anthony St Ledger and Manus O'Donnell in Cavan in 1541, during which the latter agreed to travel to England and submit to the king.

East Breifne sent soldiers to accompany Henry VIII's expedition into France in 1544 and in February 1546 during the Rough Wooing period, he offered to send his eldest son Aodh Connallach to command the forces of the English king in Scotland.

The restrictions placed on the O’Reilly kings of East Breifne by the English to rein in their power proved to be their undoing, as the kingdom's force of less than 600 men was decimated by O’Neill's army of over 5,000.

Due to the ruling O’Reilly elite's willingness to work within English institutions it was decided that East Breifne, traditionally seen as part of Connacht, was to be excluded from the first Composition of Connaught in 1577.

[33] Later that year the ailing Aodh, his son Pilib and Tánaiste Emon unexpectedly rode to Dublin with a band of horsemen to submit their lordship and apply for it to be made a shire.

On 21 August 1579 East Breifne was officially shired as County Cavan against the backdrop of separate rebellions in Munster and Ulster by James Fitzmaurice and Turlough Luineach respectively.

Sir Aodh wrote to Attorney General Lucas Dillon in November to inform him that parts of his kingdom had been looted and destroyed by English captains, and that he "being under his Majesty's laws" should not be subject to such provocations.

The country faced a potential crisis against an ascendant and hostile Tyrone and with John still in England, Aodh's second son Pilib became his lieutenant and the chief defender of East Breifne.

[34] The ability to project a show of strength was vital to the reputation of any Gaelic leader and as such, rather than depending on the lethargic Dublin government for security, he raised an army by drawing support from the military castes of East Breifne and reached out to every branch of the O’Reilly sept to unify the kingdom and steer an independent path between Tyrone and England.

John O’Reilly's government put their position on the line in their determination to cooperate fully with the English authorities and assimilate East Breifne, shired as County Cavan in 1579, into the Kingdom of Ireland in the face of severe internal opposition.

It involved huge concessions from O’Reilly but, having established himself as a shrewd diplomat with many powerful connections, John intended to resolve the internal instability of his kingdom with the agreement and secure his own position.

These territories were historically part of West Breifne and recognized the O’Rourkes as their overlords and paid exactions to them, but by the early 1500s had drifted into the sphere of the O’Reillys, who since at least 1512 had provided military aid and protection to them.

The attempts to assimilate East Breifne into English jurisdiction and law had dragged on for decades with limited progress and little to no visible benefit; even the most anglophile Breifnians had grown disillusioned with the idea.

One of the primary ways in which Gaelic leaders in the 16th century showed their strength and credibility as rulers was through the retention of a large army and possession of spoils obtained from raids on neighboring kingdoms and lesser clans.

Herbert was unknown and unconnected in Cavan and exercised his power by expanding the number of officials, advisors and assistants working for him, all of which had to be paid for through East Breifne's public taxes.

Much of county Cavan had been seized by the rebels and, with no further aid on the horizon, John and Pilib travelled north to Dungannon to submit personally to Hugh O’Neill and join the Irish alliance.

The O'Reillys had already been granted a licence to trade in the Pale in 1390, but the establishment of Cavan and its expansion as a market town under Eoghan na Feosaige greatly increased the kingdom's wealth and importance.

The Nine Years' War, which devastated not only East Breifne but all of Ireland, seems to have done lasting damage, as Arthur Chichester's 1606 tour of the county reports the existence of "a poor town bearing the name Cavan, seated betwixt many hills".

Given that the vast majority of counterfeit coins uncovered by archaeologists were found in the Cavan-Meath and Cavan-Fermanagh border area, its very likely that the O'Reilly kings condoned this practice, and possibly engaged in it.

[57] In the late 16th century it was revealed that Brian O'Rourke, king of neighboring West Breifne, had for decades paid unjust rent to the Governor of Connaught using counterfeit coins.

As the kingdom generally exerted little influence outside of its immediate borders for almost its entire existence, the clans of East Breifne included in this list are those that are specifically recorded as inhabiting the area of modern-day County Cavan, which took its present boundaries in 1579.