Fianna Fáil leader Micheál Martin has said it is outrageous that some people are saying that the right to have a mother and father is on the ballot paper or that marriage is solely defined by having children.

Speaking at the launch of his party's referendum campaign for a yes vote, he said he could not accept any argument which effectively says a married couple with no children or which can have no children is a second-class marriage.

He said he was also concerned about the impact of such an argument on lone parent families.

Mr Martin also said he did not agree with Nuala O'Loan that the Garda Representative Association should not be taking sides in the referendum.

Speaking on RTÉ's Morning Ireland, Ms O'Loan criticised GRA General Secretary PJ Stone for calling for a Yes vote in an editorial for the Garda Review.

The former police ombudsman in Northern Ireland said policing is supposed to provide an impartial and independent service.

The Mothers and Father Matter group welcome Ms O'Loan's comments.

The group said the comments speak to a bigger issue in the referendum campaign, with dissent being intimidated into silence by Yes supporters

The group said "people are simply afraid to say that they are voting no", adding "people who are voting No are worried that a vengeful Yes campaign and its supporters will cost them their jobs and their livelihoods."

Independent TD Mattie McGrath has said he expect Garda Commissioner Nóirín O'Sullivan to instruct the GRA on "the established impartial and apolitical stance of the organisation."

Mr McGrath added that if the GRA were advocating a No vote it would be met with "a storm of protest".

People Before Profit has also launched its campaign for a Yes vote in the referendum.

Speaking at the launch, Richard Boyd Barrett described it as a watershed moment for Irish society.

He said it was a once-in-a-lifetime opportunity to leave the dark past of prejudice, oppression and discrimination behind and to look forward to a future of equality and justice.

Fine Gael's Simon Coveney has also urged people to register to bote in the Referendum by tomorrow, adding the closing date for registration is Tuesday 5 May.