That sounds a bit better than it actually is. It just so happened that (1) the Irish constitution mentions marriage, that (2) changing the constitution requires a referendum, and that (3) politicians opined that there was too much case law interpreting marriage as heterosexual marriage, to fix the question just by an ordinary law.
I think you'll find precious few countries where (1) holds, to begin with; well, several US states where the religious right has introduced constitutional bans on SSM (and which have mostly by now been overturned). And that takes away the need for a referendum in most cases.
By contrast, opinion polls in Germany show consistently 70+% support for SSM, yet the main right-wing party, the CDU, until now has blocked SSM legislation. And the German constitution does not allow for referenda, neither for constitutional changes nor for ordinary laws.