A teenage boy who sexually abused his half-sister said he was copying what he saw on a pornographic website, a court has heard.

The 16-year-old, who cannot be named to protect his identity and the identity of his sister, arrived at the Central Criminal Court on signed pleas of guilty to five counts of rape and 44 counts of sexual assault, between September 2016 and January 2017.

The court heard the teenager's mother walked into her living room on 30 January 2017, to find her son who was then 14, inappropriately touching her daughter, who was then eight.

She complained to gardaí and the boy was arrested and made admissions. He said he was copying things that he had seen on a pornographic website.

In a victim impact statement, read out in court, the mother described the shock and horror she experienced upon discovering what her son had been doing to her daughter.

She said she had believed her children were protected from the outside world but her daughter's innocence had been violated under the most cruel circumstances imaginable.

She said she was grieving on two fronts - for her daughter and her lost son - and she said she was only beginning to contemplate some level of forgiveness for her son.

She said her daughter had nightmares and was worried her half-brother would come back to follow through on threats he made during the abuse and she hoped her daughter had not been damaged beyond repair.

The court heard the boy is now living with his father, is performing well in school and he does not have a smart phone. He has no previous convictions and has not come to the attention of gardaí since his arrest.

Mr Justice Michael White ordered that reports be made available to the court and adjourned the matter to 19 February.