It is assumed that the question is referring to a co-habiting couple.
Where a couple is cohabiting rather than married, they are treated as separate and unconnected individuals for the purpose of Income Tax. Each partner is a separate entity for tax purposes and, accordingly, credits, bands and reliefs cannot be transferred from one partner to the other. Cohabiting couples are expressly recognised for the purpose of social welfare law but are not recognised for the purposes of Income Tax law. Although this may appear contradictory, a key aim of both the welfare code and the tax code is to uphold the constitutional right of married couples not to be treated less favourably than unmarried couples.
The basis for the current tax treatment of married couples derives from the Supreme Court decision in Murphy v 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 and having the same income.
The treatment of cohabiting couples for the purposes of social welfare is primarily a matter for the Minister for Employment Affairs & Social Protection. However, it is also based on the principle that married couples should not be treated less favourably than cohabiting couples. This was given a constitutional underpinning following the Supreme Court decision in Hyland v Minister for Social Welfare (1989) which ruled that it was unconstitutional for the total income a married couple received in social welfare benefits to be less than the couple would have received if they were unmarried and cohabiting.