After a school career in which he often made minimal effort, he went on to Trinity College, Dublin, from which he holds an MA degree and the Higher Diploma in Education. He has worked at a large number of schools in both Ireland and Britain, notably at Hawkhurst Court (Sussex) and Rockport (Northern Ireland). He harbours a low opinion of current education and the philosophy lying behind it.

His first venture into writing occurred with Irish Christian Names (1979). His Encyclopaedia of Arthurian Legends (1991) proved highly popular, selling 120,000 copies worldwide and being translated into Japanese. His small Dictionary of Irish Myth and Legend has also sold widely. His major work on cryptozoology A Dictionary of Cryptozoology (2004) was aimed chiefly at those who could not afford the monumental works of Eberhart and Newman, yet who needed a good reference book on the subject, which is approached more from a folkloric than a zoological angle. His Handbook of Fairies (1998) is a n alphabetical guide to that subject. He has also written Cryptosup, a booklet supplementing his dictionary. He is currently working on a number of projects.

Ronan Coghlan is married with three children. He currently lives in Bangor, Northern Ireland.