The ruling was a legal first in Ireland, where homosexuality was outlawed until 1993 and gay couples are denied many rights given to married couples. Critics contend the case highlights how Ireland's conservative Catholic 1937 constitution conflicts with contemporary European norms and fails to address the reality that hundreds of gay couples in Ireland have children.
In their unanimous decision, the five judges of Ireland's ultimate constitutional authority said a lower court erred by trying to apply the European Convention on Human Rights in favor of the lesbian couple. The Supreme Court concluded that when the two are in conflict, the Irish constitution is superior to European human rights law.
In her written judgment, Supreme Court Justice Susan Denham said the lesbian couple provide a loving, stable home for their son - but that the constitution defines parents as a married man and woman, and gays are not permitted to marry in Ireland.
She said Irish law does identify the sperm donor as the father, and he therefore had a right to have a relationship with his son.
"There is benefit to a child, in general, to have the society of his father," Denham wrote. "I am satisfied that the learned High Court judge gave insufficient weight to this factor."
In April 2008, High Court Justice John Hedigan ruled in favor of the lesbian couple and rejected the man's application to have visitation or guardianship rights. The man immediately appealed.
In his ruling, Hedigan said Irish law contained nothing explicit to suggest that two women and a child possessed "any lesser right to be recognized as a de-facto family than a family composed of a man and woman unmarried to each other and a child."
Hedigan said the European rights charter's Article 8 did not discriminate between heterosexuals and gays in enshrining their right to a private family life.
The 42-year-old man, whose identity has been concealed throughout two years of legal wrangling, attended Thursday's judgment and said he was overjoyed with the verdict.
The lesbian couple did not attend.
The Supreme Court appealed to both sides to negotiate an agreement on when the man could begin visiting his son. It referred the question of granting the man full guardianship rights back to the High Court.
The man testified that the lesbian couple had been his good friends, and he agreed to donate sperm to one of them on condition that he would be treated as the family's "favorite uncle."
But after the 2006 birth, both sides agreed that their relationship soured - reaching the breaking point when the two women decided to move to Australia with the boy.
The man successfully sued to prevent them from leaving Ireland pending a custody ruling.
Ireland's parliament has yet to pass laws that effectively regulate fertility clinics or define the clashing parenting rights of gay couples versus sperm donors.
Earlier this month, the government opened debate on a Civil Partnership Bill that, if passed, would give gay couples many marriage-style rights, particularly in relation to property, finances and inheritance. It offers no legal recognition of their right to be parents.