Mr Justice Paul Carney said it was one of the worst cases of abuse of a daughter one could possibly find and it was at the top of the scale as regards offences.
However, he said he had to consider the health position of the accused.
He said he was aware that if he imposed a severe custodial sentence and suspended it, it would go out to the community in soundbites that in one of the most serious cases of serial rape of a daughter, the accused walked.
However, he added it was one of the most difficult cases to resolve because on the other side, if he imposed a substantial custodial sentence unsuspended, because of O'Brien's health problems, it would be branded that he had substituted one injustice for another.
The judge referred to another case where the Court of Criminal Appeal suspended in total moderate sentences imposed because of the state of health of the convicted individual.
He sentenced O’Brien to 12 years in prison, suspended the final nine and immediately granted him continuing bail pending an appeal.