http://freepages.genealogy.rootsweb.com/~mallorybrody/Eire/Clans_and_Families_of_Ireland_and_
Scotland_The_Gaels.html

Clans and Families of Ireland and Scotland

The O’Hagans (O hAgain) of the Cineal Eoghain descend from Tighearnach, who was a son of Muireadhach mac 
Eoghain, and thus a grandson of Eoghain, the eponymous ancestor of the clan. They were divided into two groups: 
The main being chiefs of Cineal Fearghusa, a territory around Tullaghoge or Tullahogue in County Tyrone 
(Tir Eoghain), and the other being chiefs of Cineal Tighearnaigh in County Derry, where their presence is recalled 
by the place called Ballyagan (there is another Ballyhagan in Antrim). It was the hereditary privilege of the O’Hagans 
to inaugurate The O’Neill at their seat of Tullahogue (along with the O’Cahans). 

http://members.aol.com/lochlan2/odugan.htm

The Topographical Poems of John O'Dubhagain
and Giolla Na Naomh O'Huidhrin

Edited by John O'Donovan 1862
Printed for the Irish Archaeological and Celtic Society, Dublin 


NOTES TO THE PART OF O'DUGANS POEM RELATING TO ULSTER

74. Oh-Ogain, now O'Hagan. The site of the ancient residence of O'Hagan is to be seen on a gentle eminence a 
short distance to the east of the village of Tullaghoge. It is a large circular fort, surrounded by deep trenches and 
earthen works.

75. Another O'Hogan - this family is unknown. there are some families of this name in the counties of Tyrone and 
Londonderry, supposed to be different from the O'Hagans, but their history is forgotten.

http://www.goireland.com/genealogy/scripts/Family.asp?FamilyID=162

"the usual form in Ulster is now O hAgain in Irish, which was originally O hOgain"