Joseph O'Callaghan published perhaps the most comprehensive genealogy book I have seen about the Callahan family, which represented a lifetime of his own research. The book was entitled, The O Callaghan Family of County Cork, A History. He has since published a revised edition in 2020 and also written a fiction book about a Callaghan family in the 17th century. The novel is about Donough and Ellen O'Callaghan whose lands were confiscated and they were transplanted to County Clare. He is currently as of 2023 working on a book about Callahan sites one can visit in Ireland.
To order a copy see:
Clan Callaghan: The O Callaghan Family of County Cork. Revised edition. Baltimore: Clearfield Company/Genealogical Publishing Company, 2020.
and
Land, Loved and Lost. A Tale of Love and War in 17th Century Ireland. Carrigtwohill: Printmybook.com, 2022
Joe O’Callaghan is a retired professor of medieval history from Fordham University, New York. He published many books about Medieval Spain. The description of the book about the O'Callaghans (from the Irish Family Names of Dublin website) is as follows:
The O Callaghan Family of County Cork: A History. By Joseph F. O' Callaghan – The book is a 284 pg Hardback with numerous illustrations (black & white). Size: 12 inches x 8.5 inches.
"This extremely well researched history of a County Cork sept traces its
origins from Cellachán of Cashel, the progenitor of the Cork O Callaghans,
down to modern times.
Cellachán was the King of Munster in the middle of the 10th century. Two
great County Cork septs descended from Cellachán, the O Callaghans and the
MacCarthys.
It was not until the 16th century that factual detail began to emerge and
historians were able to get a clearer picture of the territory occupied by
the sept and the names of the clan chieftains. At that stage the entire
Gaelic way of life had come under severe pressure and the land ownership
was passing into the hands of the adventurers, comprised of New English and
Anglo-Irish speculators.
By the middle of the 17th century the lands of the Gaelic, Catholic O
Callaghans had been confiscated and many of the great lords now became the
tenants of the new masters. Some managed to survive the Penal Laws of the
18th century and became the owners of vast estates in Clare and in
Tipperary. For the majority the transition was painful and they and their
descendants had to endure servitude and poverty until the Land Acts of the
late 19th and early 20th century, wrung from a reluctant English
government, restored their dignity and paved the way for future prosperity.
In his chapter entitled The O Callaghan Diaspora the author mentions many O
Callaghans worldwide who have become very successful people but of course
they represent only the tip of the iceberg. The many pages of genealogical
tables will be of interest to many.
What is remarkable about the O Callaghans is the fact that despite the
hundreds of years of penury and subjection their native resilience and
intelligence has enabled many proud bearers of the name to rise to the top
in practically all areas of human endeavour."