According to Whitaker, this county was inhabited in the time of Ptolemy by the Scoti, who then possessed all the inland parts of Ireland: it afterwards formed part of the district of Uriel, Oriel, or Orgial, which also comprehended Louth and part of Armagh; but it was more generally known by the name of Mac Mahon's country, from the powerful sept of that name. Its present name is derived from its chief town, Monaghan or Muinechan, "the Town of the Monks," although no trace of an ecclesiastical establishment can now be discovered there. Immediately after the English invasion, when De Courcy entered Ulster, he was joined by a chieftain named Mac Mahon, who ingratiated himself so much with him that he was entrusted with the command of two forts, which, on the first change of fortune, Mac Mahon utterly destroyed; and when questioned on his breach of faith, answered, "that he had not engaged to keep stone walls; and that he scorned to confine himself within such cold and dreary enclosures, while his native woods were open for his reception and security." Hugh de Lacy, some time after, invaded Monaghan and burned the town and abbey, but soon after erected a castle there and restored the monastic institution. In the reign of Hen. IV., Lord Thomas of Lancaster, his son, having gone to Ireland as Lord-Lieutenant, received the homage of several of the native chieftains, among whom was Mac Mahon, who then submitted so far to the rules of English law as to accept an estate for life in that part of the county called the Ferney, for which he paid ten pounds a year chief-rent. This state of acquiescence, however, was not permanent; for, in the very next reign, Lord Furnival, who was then Lord-Deputy, found it necessary to undertake a military expedition against the Mac Mahons and other insurrectionary septs in Ulster; but, though he succeeded so far as to make them sue for the king's peace, he Was unable to reduce them to the obedience of subjects. The county remained in the same state until the time of Elizabeth, in the 11th of whose reign, the parts of Ulster that had not previously acknowledged the Queen's authority, were reduced into seven shires, of which Monaghan was one; and afterwards the Lord-Deputy Fitzwilliam, during a progress through this part of Ulster, caused Mac Mahon to be attainted and executed for high treason, and the county to be divided according to the baronial arrangement which it still retains, the lands to be allotted among the Irish occupiers and English settlers, and to be held according to the tenures of the law of England. According to this arrangement, the particulars of which are still extant in the original document, the five baronies contained one hundred "ballibetaghs," a term applied by the Irish to a tract of land sufficient to maintain hospitality, each ballibetagh containing 16 tathes of 120 English acres each; thus making the area of the county 86,000 acres, exclusively of church lands. All the grants then made contained a clause of forfeiture, in case of the re-assumption of the name of Mac Mahon, of failure in payment of rent, or of attainder on rebellion. The subsequent insurrection of the Earl of Tyrone, however, prevented the plan from taking effect. The chief of the Mac Mahons still continued to arrogate the title of supreme lord, and the whole county was occupied by three or four families only, namely, those of the chieftain, and of Mac Kenna, Mac Cabe, and O'Conally. So little had the progress of civilisation been forwarded by the measures of the English government, that in the succeeding reign of Jas. I., when the lord-deputy made a progress thither to inspect and settle the province, he was forced on entering the county to encamp in the open field. On investigating the titles by which the lands were held, it was found that the patents were all void in consequence of the non-observance or breach of some of the conditions, new grants were therefore made, and the country being reduced to a state of perfect submission, partly by intimidation and partly by concession, continued tranquil till the war broke out in 1641, when it followed the example of the rest of the north of Ireland in joining with the Irish against the lately established government, and the Mac Mahons again vainly endeavoured to recover their supremacy.
Transcribed from A Topographical Dictionary of Ireland, 1840 by Samuel Lewis