While the social welfare treatment of married and cohabiting couples, under social assistance schemes, is primarily a matter for the Minister for Social Welfare, it appears that the Supreme Court held in the Hyland case in 1989 that it was unconstitutional for the relevant social welfare provisions to treat a married couple living together less favourably than an (unmarried) cohabiting couple. This judgment has been given effect by treating cohabiting couples in the same way as married couples for social welfare purposes.
In relation to taxation, there are no special income tax allowances for unmarried couples living together. In this context, I would point out that tax law follows the general law relating to marriage. The basis on which the married person's tax free allowance (and double rate bands) is given derives from the Supreme Court decision in Murphy vs The Attorney General (1980) which held that it was contrary to the Constitution for a married couple to pay more tax than two single people living together.