The High Court in Dublin has ruled that a gay couple can challenge the Irish tax authority's refusal to treat them as a married couple.
Louise Gilligan and Catherine Zappone at Dublin's High Court
Ann Louise Gilligan and Katherine Zappone wed in Canada last year.
When they got back home to Brittas in County Dublin they asked the tax authorities if they could file their returns as a married couple.
But the Revenue Commissioners rejected their application, which would have meant they paid less tax.
High Court judge Mr Justice Liam McKechnie, who sits on Ireland's second-highest court, said on Tuesday they will get a full hearing to seek a judicial review.
He said the case was not simply about tax bands or treatment of married or unmarried people, but had profound implications for same sex marriages and for society as a whole.
Ms Zappone is on Ireland's government-appointed Human Rights Commission and Ms Gilligan is a philosophy lecturer.
Their lead lawyer, Gerard Hogan, argued that neither Ireland's 1937 constitution nor its more recent tax laws explicitly defined marriage as solely between a man and a woman.
Mr Hogan, one of Ireland's most prominent experts on constitutional law, said the Revenue Commissioners "have discriminated against them in an unjust and invidious manner, in breach of their constitutional rights and the European Convention on Human Rights".
He conceded that the Irish Constitution undoubtedly presumed that marriage meant between a husband and wife, but argued that constitutional law should not be
trapped within "the permafrost of 1937".
The case could have major implications for unmarried couples in the Republic of Ireland.
The 2001 census recorded 77,600 households of unmarried partners with 1,300 being gay couples. Unmarried couples pay higher income and inheritance tax.