A prominent campaigner for a Yes vote in the forthcoming same-sex marriage referendum has said she is "worried" about the outcome of the vote.

Independent Senator Katherine Zappone today described the No side's campaign as "very clever" and "very challenging".

Speaking on her experience canvassing for a Yes vote, Ms Zappone said: “I’m worried because it’s hard for a good number of people I’m talking to to actually move beyond that traditional concept of family.”

However, Ms Zappone said she believed the Yes side would win on May 22nd if people’s concerns were addressed.

Ms Zappone said: “It’s no longer time to be giving out pat messages. It’s about hearing people’s concerns.

“The very small number of same-sex couples who will ultimately or currently have children will do just as good a job.”

‘Not comfortable’

The senator said that some people were “not that comfortable” with the concept, and the question was, “are they willing to move beyond the traditional understanding of the family?”

However, Ms Zappone said: “It’s good to put it up to us. I do believe we will win and can win.”

Ms Zappone’s comments came after Tánaiste Joan Burton said that some of the posters that had been erected by the No campaign were “sad and demeaning”.

Former minister for justice Alan Shatter also entered the debate on the referendum this week, claiming that the No side were using children as weapons.

Unregistered voters have until 5pm today to drop forms into their local councils, after getting them signed and stamped at local Garda stations.

Voters will be asked on May 22nd if they agree with the proposal to amend the Constitution contained in the Thirty-Fourth Amendment of the Constitution (Marriage Equality) Bill 2015.