This, and more importantly, the fact that the "family" bit that's always quoted doesn't actually use that word in Irish (it uses 'household' instead, and remember the Irish bit has legal precedence) have been argued a number of times.
It has never been tested before the SC and I'm not sure any government is willing to risk it, either. Would need an attempt to legislate and someone to appeal it, basically.
Previous cases haven't actually tested the "family" section, in Irish, which is the critical point.
The High Court ruled previously that the ethos of the document (Bunreacht na hÉireann) would infer that the meaning of marriage as written is specific to the marriage of a man and a woman (given the earlier references to Catholic sentiment and the outlay referencing God in a Catholic manner). This was the finding of Judge Elizabeth Dunne in 2006 after a case was brought by Senator (then simply Dr.) Katherine Zappone and Dr Ann Louise Gilligan after they had legally been married in Canada for almost 30 years. I believe that they were due to appeal this to the Supreme Court this year (last month if I'm not mistaken) but I haven't heard anything since. If the Supreme Court sides with the High Court ruling (very likely given the lack of new evidence) then the referendum would in fact be needed.
Think the government should just get over it and call the referendum so we can bring Ireland into the 21st Century with a Yes vote!
If you read the Dunne judgment you'll see that she acknowledges the Constitution as a living document such that the definition of marriage can change over time. In her opinion the definition of marriage simply hadn't changed enough, as of 2006.
Notably, since 2006 we have seen a bunch of jurisdictions come to recognize gay marriage - the number of people living in jurisdictions which recognize gay marriage has almost doubled since 2006 - and the research on outcomes for children raised by same sex couples has grown and bolstered the scientific consensus that gender-combination is irrelevant when it comes to raising children. Both of these issues were cited by Dunne in her judgment.
More importantly- the Dunne judgment sought to answer the question "does the Irish Constitution require that recognition be given to same-sex marriages" which is not the same as the question "does the Irish Constitution prohibit recognition of same-sex marriages?"
We only need a referendum if the Constitution prohibits same-sex marriages. Whether it does or not would be easy to test- legislate for gay marriage, let a couple get married, and test the law before the Supreme Court.
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