Surely this is unconstitutional to provide for some and not for others?
No, its actually the opposite - they have to be discriminatory to conform with the provisions in the Constitution.
Due to the special place of the family (married in the case of couples + children) in the Constitution, in tax and social welfare dealings with the State, the co-habiting couple must always be in a less favourable position than the married couple.
So for taxation, co-habiting are treated as single as this is less favourable.
And for Social Welfare, co-habiting are treated as couples as this is less favourable.
Under Irish law, married people have legal responsibilities and obligations that single people dont have, so if the Government were to give both married and co-habiting people the same benefits, it would be discriminatory against the married people as "benefits + responsibilities" is less favourable than "benefits".
You could look at it another way. Co-habiting couples are paying for the additional freedom and lack of obligation/responsibility that they have in comparison with married couples.
Remember that the obligations and responsibilities that married people are a lot wider than living costs - they have inheritance, family law issues, next of kin, divorce issues etc. that co-habiting couple dont have.