A teenager has been arrested in connection with the incident which happened yesterday afternoon.
Gardai say an altercation broke out between the two men in a house at Kilcaroon near the village of Ballyporeen shortly before 2.45 yesterday.
A 66 year old man suffered serious head injuries in the incident – he was taken to South Tipperary General Hospital and later transferred to Cork University Hospital.
An 18 year old youth was later arrested by Gardai in Cahir – he can be held for up to 24 hours.
Investigations are ongoing – witnesses or anyone with any information is asked to contact the Gardai in Cahir on 052 7445630.