The Social Welfare and Pensions Bill has passed all stages in the Seanad by 26 votes to 17.
The Bill gives effect to changes in allowances to lone parents announced in the Budget.
Opposition Senators sought to defer or remove the measure that would see the allowance end when the youngest child reaches seven years until there is a satisfactory childcare package in place.
The legislation is being signed into law tonight following the passage on an early signature motion in the Seanad.
It will be signed by the Presidential Commission in the absence of President Michael D Higgins, who is in the US on an official visit.
The commission comprises Dáil Ceann Comhairle Sean Barrett, Seanad Cathaoirleach Paddy Burke and Chief Justice Susan Denham.
Minister for Social Protection Joan Burton told the Dáil that she would not introduce the measure until she got "bankable" guarantees from the Government that an affordable childcare package would be in place.
Minister Burton told Senators that work had already begun between a number of Government departments on the provision of childcare, which she said would build on existing childcare measures.
Opposition Senators expressed doubt in the absence of some form of guarantee written into the legislation.
She reminded Fianna Fáil that the free pre-school year it introduced was announced in a Budget without detail.
Minister Burton said the legislation was about improving outcomes for lone parents and their children.
She said over €1bn was being spent on allowances for one-parent families, but the outcome in terms of reduction of risk of poverty was "nothing like it ought to be".