The distribution map is in good agreement with the historical account of the Hanlons. The majority of the Hanlons are found in southern County Armagh and northern County Louth. Some accounts claim that the O'Hanlons originally occupied a territory farther north in County Armagh (the Barony of Oneilland East) - and that they were driven south to the Barony of Orior by other invading clans just before the Norman invasion (mid-1100's).
There is also a large group of Hanlons in County Cork. The historical account claims they arrived there only in comparatively recent times - but that seems highly unlikely due to their presence in widely scattered remote areas of that county. More likely there was some completely unrelated Irish family in Co Cork whose name was also anglicized to Hanlon.
A total of 1241 Hanlons were listed in the Griffith Valuation. This means that about one out of every 800 Irishmen was a Hanlon.
There were only 80 O'Hanlons listed in the Griffith Valuation (mostly in Counties Armagh and Louth). Presumably most O'Hanlons shortened their name to Hanlon sometime in the past.
The maps below show the distribution of Hanlons at the civil parish level: