From A Topographical Dictionary of Ireland, 1837
CAVAN (County of), an inland county of the province of ULSTER, bounded on the north by the county of Fermanagh; on the west, by that of Leitrim; on the south, by those of Longford, Westmeath, and Meath; and on the east and north-east, by that of Monaghan. It extends from 53° 43' to 54° 7' (N. Lat.); and from 6° 45' to 7° 47' (W. Lon.); and comprises, according to the Ordnance survey, 477,360 statute acres, of which 421,462 are cultivated land, 30,000 unimproved mountain and bog, and 22,141 are under water The population, in 1821, was 195,076; and in 1831, 228,050.
According to Ptolemy, this tract, with the districts included in the adjacent counties of Leitrim and Fermanagh, was occupied by the Erdini, designated in the Irish language Ernaigh, traces of which name are yet preserved in that of Lough Erne and the river Erne, upon which and their tributaries these districts border. This district, exclusively of the greater part of the present county of Fermanagh, formed also the ancient principality of Breghne, Brefine, Breifne, Breffny, or Brenny, as it has been variously spelt, which had recognised limits from time immemorial, and was divided into the two principalities of Upper or East Breifne and Lower or West Breifne, the former composed almost entirely of the present county of Cavan, and the latter of that of Leitrim. East Breifne was often called Breifne O'Reilly, from its princes or chiefs having from remote ages borne that name: they were tributary to the O'Nial of Tiroen long before the arrival of the English, although Camden says that in his time they represented themselves as descended from the English family of Ridley, but were entirely Irish in manners.
The county is celebrated in the history of the wars in Ireland for the fastnesses formed by its woods, lakes, and bogs, which long secured the independence of its native possessors. Cavan was one of the counties formed in Ulster, in 1584, by Sir John Perrott, lord-deputy of Ireland, and derived its name from the principal seat of its ancient rulers, which is still the provincial capital: in the following year it was represented in a parliament held in Dublin by two loyal members of the family of O'Reilly. Both Breffnys anciently formed part of Connaught, but the new county was incorporated with Ulster. The O'Reillys were at this time a warlike sept, particularly distinguished for their cavalry, and not living in towns, but in small castles scattered over the country. In order to lessen their influence by partitioning it among different leaders, and thus reduce them to the English law, it was resolved to divide the country into baronies and settle the proprietorship of each exclusively on a separate branch of the families of the former proprietors.
Sir John O'Reilly, then chief lord of the country, had covenanted to surrender the whole to Queen Elizabeth, and on the other part Sir John Perrott had covenanted that letters patent should be granted to him of the whole; but this mutual agreement led to no result, and commissioners were sent down to carry the division into effect. By them the whole territory was partitioned into seven baronies, of which, two were assigned to Sir John O'Reilly free of all contributions; a third was allotted to his brother, Philip O'Reilly; a fourth to his uncle Edmond; and a fifth to the sons of Hugh O'Reilly, surnamed the Prior. The other two baronies, possessed by the septs of Mac Kernon and Mac Gauran, and remotely situated in the mountains and on the border of O'Rorke's country, were left to their ancient tenures and the Irish exactions of their chief lord, Sir John, whose chief-rent out of the other three baronies not immediately possessed by him was fixed at 10s. per annum for every pole, a subdivision of land peculiar to the county and containing about 25 acres: the entire county was supposed to contain 1620 of these poles.
But these measures did not lead to the settlement of the country; the tenures remained undetermined by any written title; and Sir John, his brother, and his uncle, as successive tanists, according to the ancient custom of the country, were all slain while in rebellion. After the death of the last, no successor was elected under the distinguishing title of O'Reilly, the country being broken by defeat, although wholly unamenable to the English law.