Fianna Fáil senator Jim Walsh is understood to have resigned the party whip over his opposition to the Children and Family Relationship Bill.
Mr Walsh, who is among the conservative wing of Fianna Fáil, made the decision this afternoon.
There had been speculation that the Wexford politician would be stripped of the whip due to him opposing the party's stance on the landmark bill.
But Mr Walsh took colleagues that he has in fact "resigned" the whip. He made the decision after party leader Micheal Martin.
A party spokesman told Independent.ie that Mr Walsh's views on the matter are "not consistent" with those held by the party.
The news comes as the party suffered a further drop in support, according to an IPSOS/MRBI opinion poll commissioned by the Irish Times.