Marriage is for straight people too!
Both the Irish and English versions of the referendum wording are, in my view, open to the interpretation that two persons who are of different sex from each other may still marry each other if the amendment is passed. While the amendment anticipates that they may be of the same sex, they do not have to be. Arguably, the wording could have been more explicit on this point. Yet even if you accept that there is some potential ambiguity here, the risk that the amendment will inadvertently ban marriage for opposite-sex couples is non-existent.
Even if the amendment could be read as applying to same-sex couples only, it is exceptionally unlikely – without very clear wording to that effect - that it would displace the current right of opposite-sex couples to marry.
A vitally important point to remember is that, when it comes to its meaning or interpretation, the Constitution is not like every other law. The Constitution is a broad, political document, more beholden to rhetoric than precise legal formulations. As legal documents go, it tends to be quite sparing and skeletal. The kind of detail and precision you might expect to find in, say, the Taxes Consolidation Act or the Companies Act is generally absent from the Constitution. Certainly, the Constitution is prescriptive and clear on several points (such as the grounds for divorce, which are integrated into the constitutional text). Usually, however, the Constitution tends to deal in general principles and leave the fine detail to legislation.
When determining the meaning of the Constitution and amendments to it, the Courts lean heavily against any overly literal approach. For instance, in O’Byrne v Minister for Finance  IR 1, the Supreme Court rejected an argument that a ban on diminishing the remuneration of a judge while in office was infringed by the taxation of a judge’s salary. On a literal interpretation one might feasibly say taxes reduce or diminish a person’s salary. The Court, however, looked to the purpose of the measure, and found that taxation of judges’ salaries on the same basis as applies to any other citizen was perfectly legal. In the Attorney General v Paperlink  ILRM 373, Costello J noted that the Constitution is a broad, political as well as legal document. He noted that it is not to be parsed in the same manner as a Finance Act but should instead be read in the light of its overall purpose. He thus rejected the proposition that minor textual differences between two sub-articles dealing with human rights meant that the two sub-articles provided different levels of protection. Most instructively, in Roche v Roche  IESC 82 some of the judges of the Supreme Court looked to the context and purpose of the 1983 abortion referendum in determining that embryos created in a fertility clinic did not fall within the scope of the term 'unborn' in the Eighth Amendment unless and until they were implanted in the womb. The amendment's purpose - to prevent any attempt to legalise a termination of pregnancy where there was no risk to the life of the mother - heavily influenced the Court's ruling.
Marriage for opposite-sex couples is well-established in our legal order. The right to marry has been recognised by the courts as a constitutional right, albeit only for heterosexual couples. Any attempt to abolish that right would require a very clear and explicit wording that unequivocally rendered heterosexual marriage unlawful. An ambiguous wording would not be enough in itself. No reasonable judge would declare heterosexual marriage unconstitutional without a very clearly expressed, unambiguous constitutional wording to that effect.