Ballybay Moffetts: a brief history of the Crievagh House,
County Monaghan, Ireland, branch of the Moffett family. Free to download.

Clogher Clergy and Parishes, J. B. Leslie
Free to download in various formats.

History of Monaghan for 200 years (1660-1860)
Historical Sketches of County Monaghan and Monaghan in the Eighteenth Century by Denis Carolan Rushe. Free to download.

Irish Names of Places, 3 vol.s, PW Joyce
The Internet Archive site has all three volumes, free-to-download, .pdf format.

Post Offices of County Monaghan
Klaus Stange produced an excellent book on this subject in 2005, extracts available online. Clogher Historical Society holds a copy in its library.

Record of the Smith family descended from John Smith
born 1655 in county Monaghan, Ireland. Free to download.

Royal Historical Society Bibliography
Online guide to British and Irish history from the Roman period to the present day. Hosted by the Institute of Historical Research, University of London.

Sacred Text Archive
Archive of online books about religion, mythology, folklore and the esoteric. The site is dedicated to religious tolerance and scholarship. Includes, for instance, The Religion of the Ancient Celts by J. A. McCulloch (1911).

Some account of the territory or dominion of Farney
By EP Shirley. Free to download.

Statistical Survey of County Monaghan, 1801
Google Books has produced a free-to-download .pdf version of Charles Coote's survey.