Richard Brennan, aged 64, who is originally from Rathfarnham but has been working in the United States, pleaded guilty during his trial to raping and indecently assaulting his sister Paula Fay when she was between 15 and 17 years old.
He also pleaded guilty to raping and indecently assaulting Catherine Wrightstone when she was between nine and 13 years old and he admitted indecently assaulting a third sister, Yvonne Crist, when she was 20 and he was 18.
He changed his plea to guilty after all three of the sisters had given evidence at the Central Criminal Court and two of them had been cross-examined.
The sisters have waived their right to anonymity to allow Brennan to be named.