According to Ptolemy, this tract, together with that comprised in the counties of Fermanagh and Cavan, was occupied by the Erdini, called in Irish Ernaigh, who possessed the entire country bordering on Lough Erne. This county, together with that of Cavan and part of Fermanagh, afterwards formed the territory of Breffny or Brenny, which was divided into two principalities, of which the present county of Leitrim formed the western, under the name of Lower or West Breffny, and Hy-Briuin-Breffny, from Brian, son of Eachod, and grandson of Muredach. first king of Connaught of the Scottish race. Sometimes this county was also designated Breffny O'Ruark, O'Rorke, O'Roirk, or O'Rourk, from the name of the family that ruled over it from a very early period. Its subordinate divisions were Dromahaire, the present barony of the same name; Lietdrumai or Liathdromen, the modern Leitrim; Munster Eolus, or Hy Collning, the present baronies of Carrigallen and Mohill, the principal families of which were the Maghrannals, or Mac Granells; and Hy Murragh, the modern barony of Rossclogher, of which the chiefs were the O'Murroghs, or O'Murreys. For some time after the arrival of the English, the whole was considered to form part of the ill-defined county of Roscommon : but the O'Rourks maintained an independent authority in their own territory until the middle of the 16th century. Tiernan O'Rourk, an active military chief, governed here in the latter part of the 12th century, when the princes of Connaught and Leinster combined to expel him from his territory; and Dermod Mac Murrough, the king of Leinster, taking advantage of their success, carried off his wife Dervorghal; but the expelled chieftain having applied for aid, to Turlogh, supreme king of Ireland, the latter not only reinstated him in his principality, but regained him his wife. The English, soon after their arrival, in conjunction with their ally Dermod, invaded the territory of Breffny, where, however, Dermod was twice defeated, and compelled to secure his safety by a precipitate retreat. O'Rourk afterwards made an unsuccessful attack on Dublin, when in the possession of Strongbow's forces; yet subsequently he joined Hen, II. against Roderic, king of Connaught. The line of independent chieftains of this family terminated in Brian O'Rourk, lord of Breffny and Minterolis, who, relying on the promises of Pope Sixtus V. and the king of Spain, threw off his allegiance to Queen Elizabeth; but having been forced to flee to Scotland, he was there taken prisoner and conveyed to London, where he was executed as a traitor, on which occasion it is recorded that the only favour he asked was to be hanged, after his country's fashion, with a rope of twisted withe. His territory having escheated to the Crown, extensive grants were given to English proprietors, and, in 1565, it was erected into a county by Sir Henry Sidney, under the name of Leitrim, from its chief town. The O'Rourks ruled over several subordinate septs, the principal families of whom were the O'Murrey's, Mac Loghlins, Mac Glanchies, and Mac Grannels, some of whose posterity still exist; the descendants of the lastnamed family are now called Reynolds, a corruption of the original name.
Transcribed from A Topographical Dictionary of Ireland, 1840 by Samuel Lewis