In earlier times the townland was probably uninhabited as it consists mainly of bog and poor clay soils. It was not seized by the English during the Plantation of Ulster in 1610 or in the Cromwellian Settlement of the 1660s so some dispossessed Irish families moved there and began to clear and farm the land.
A lease dated 17 September 1816 John Enery of Bawnboy includes Carnacliff otherwise Carnacligh.
The landlord of Cornacleigh in the mid-19th century was John Finlay.
Records of inhabitation include:
The Tithe Applotment Books for 1826 list nine tithepayers in the townland.
The Cornacleigh Valuation Office Field books are available for September 1839.
In 1841 the population of the townland was 60, being 29 males and 31 females. There were nine houses in the townland, all of which were inhabited.
In 1851 the population of the townland was 47, being 22 males and 25 females, the reduction being due to the Great Famine (Ireland). There were seven houses in the townland, all inhabited.
Cornacleigh lies in the Roman Catholic parish of Corlough.
Cornacleigh National School
The roll number was 8713. In 1862 Michael McAuley, a Roman Catholic, was the headmaster. There were 88 pupils, all Roman Catholic. The Catechism was taught to the Catholic pupils on Tuesdays and Thursdays from 3pm to 3:30pm and on Saturdays from 10am to 12 noon.
1874: It was changed to an Ordinary Agricultural School which meant a national school with a small farm attached. One male teacher who received an annual salary of £37. There were 82 pupils, 33 boys and 49 girls.
