A young Irish couple have revealed how they fell in love and had a child - then discovered they were half-brother and sister.

The couple, who spoke to the Irish Mail on Sunday on the condition of anonymity, said they were stunned but vowed to stay together and have more children.

"I'm not blind and I'm not oblivious to what people might think," the young man involved in the relationship told the paper. "We're not from a bad background and if someone had said to me that they were in a relationship like this, I would have said they were sickos."

The couple, both in their 20s and living in the province of Leinster, were from different towns but met in a nightclub several years ago.

Two years after they met, the woman fell pregnant and the pair moved in together, the paper reported.

The man had not seen his father - or who he had been brought up to believe was his father - since he was a young boy and had a strained relationship with his mother, until he began to make amends last Christmas.

"My mother was by herself and I was just going to call in and see her before Christmas," said the man. "I started to tell her about my girlfriend and our child."

On hearing who the woman's parents were, "My mother got hysterical," said the man. "She just put her hands on her face and said: 'You're not serious.' I thought: 'She has lost it.'"

Several days later, the man's mother finally told him the truth - his girlfriend's father was his father too.

Initially he refused to believe it. "But somehow in the back of my mind I knew it was true," said the man. "When I was growing up, I always knew something was being held back."

With the help of DNA tests and "gentle questioning", the couple now know the truth. During a month-long relationship, the man's mother had become pregnant to the woman's father. They were half-brother and sister.

About four years after the man was born, his real father discovered the woman he had a fling with had had his child. He began a legal battle to gain access to his son, reported the paper.

The court decided that although he was the boy's real father, he would not be allowed access and his son would not be allowed to know the truth.

The brother-sister couple are now considering taking a landmark civil case against the psychologist and judge who made the fateful decision.

- NZ Herald Staff