In the old Gaelic system of land division, Monaghan was part of the Kingdom of Oriel. lt was also known as McMahon's country after the dominant family of the area. The McMahons and their allies, the McKennas and O'Connollys, maintained effective domination of the county even after the arrival of the Normans in the twelfth century. The county boundaries were not established by the English administration until the late sixteenth century.

After the defeat of the rebellion of O'Neil and the Ulster chieftains in 1603, the county was not planted like the other counties of Ulster. The lands were instead left in the hands of the native chieftains. In 1641 the McMahons and their allies joined the general rebellion of Irish Catholics, and following their defeat there was some plantation of the county with Scottish and English families.

Analysis of the Hearth Money Rolls of 1663 shows that the commonest names in the county at the time (in descending order) were McMahon, McKenna, O'Duffy, O'Connolly, McCabe, McWard, McArdle, McIlmartin, O'Byrne, O'Callan, McCallan, O'Kelly, O'Murphy, McNancy, McTreanor, O'Gowan or McGowan, O'Boylan, McIlcollin, O'Finnegan, O'Cassidy, and McPhilip.

The major settlers in the county were Scottish farmers brought over from the area of Strathclyde. Common names among these settlers were McAndrew, Mackay, Sinclair, Stewart, Buchanan, McKenzie, Davidson, Ferguson, Blackshaw, McCraig, Walker, Cameron, Gordon, Patterson, and McCutcheon.

A general indication of the proportion of the population of Irish or Norman extraction, or of English or Scottish descent, can be derived from the statistics on religious persuasions of the inhabitants. These groups were, respectively, predominantly Catholic, Church of Ireland, or Presbyterian. In 1861, when the census first determined religion, the respective proportions were 73, 14, and 12 percent."
From Pat Traynor, in California's Gold Rush country

1659 Census: In the census of 1659, the total population of Monaghan was recorded as 4083 - 3649 Irish and 434 English.