Ireland’s Supreme Court ruled that, while the man was not entitled to guardianship, he has natural rights over the three-year-old child.
Five judges unanimously found it was in the best interest of the boy’s welfare to remain in contact with his biological father.
The court previously heard that the man donated his sperm to the lesbian couple, who were his friends, so one of them could have a baby.
It was the couple’s preference that the child would have knowledge of his biological father while the donor - a 41-year-old gay man - would be like a “favourite uncle”.
But their friendship deteriorated and he started court action two years ago when the couple revealed they planned to move to Australia for a year with the boy.
The Supreme Court overturned an earlier High Court decision that the man was not entitled to access to the child.
In her judgment, Ms Justice Susan Denham found that the sperm donor has rights as a natural father and added that he had formed a bond with the child when he was born.
“There is benefit to a child, in general, to have the society of his father,” she said.
“I am satisfied that the learned High Court judge gave insufficient weight to this factor.
“The basic issue is the welfare of the child,” she added.
The judge urged the parties to agree to terms of access before the case is dealt with back in the High Court.