Article 40.3.3 of the Constitution (‘the 8th Amendment’) makes it impossible for any new law to be introduced that would make abortion available in Ireland except where a woman’s life is at risk. Legal change requires constitutional change. This can only be done by a referendum.
The 8th Amendment only allows for lawful abortion in Ireland where there is a “real and substantial risk to the life, as distinct from the health”, of the pregnant person (Attorney General v X  1 IR 1, p. 53).
All laws introduced by the Oireachtas, and all activity undertaken by the State, must be constitutional. Any law that is found to be unconstitutional can be struck down, or invalidated, by the courts; that is a key function of the courts in Ireland.
If the Oireachtas is to have the power to make new law on abortion, ‘the 8th Amendment’ (Article 40.3.3; see explanation here) must be removed. The only way in which the Constitution can be changed is by a referendum. This is laid down in Article 46 of the Constitution.
So, because the Constitution prohibits the legalisation of abortion in anything other than life-threatening situations, and since the Constitution can only be amended by referendum, a referendum must be held if the Oireachtas is to be empowered to make any liberalising changes to abortion law in Ireland.
- Any provision of this constitution may be amended, whether by way of variation, addition, or repeal, in the manner provided by this article.
- Every proposal for an amendment of this constitution shall be initiated in Dáil Éireann as a Bill, and shall upon having been passed or deemed to have been passed by both Houses of the Oireachtas, be submitted by referendum to the decision of the people in accordance with the law for the time being in force relating to the referendum.
- Every such Bill shall be expressed to be “an act to amend the constitution”.
- A Bill containing a proposal or proposals for the amendment of this constitution shall not contain any other proposal.
- A Bill containing a proposal for the amendment of this constitution shall be signed by the President forthwith upon his being satisfied that the provisions of this article have been complied with in respect thereof and that such proposal has been duly approved by the people in accordance with the provisions of section 1 of article 47 of this constitution and shall be duly promulgated by the President as a law.