The Government lost a vote 27-23 in the Seanad tonight on private members' bill proposed by Senator Feargal Quinn dealing with upward-only rent reviews.
Labour Party chief whip Emmet Stagg is to seek meetings with four Labour senators who were not present for the vote.
Senators John Kelly, Denis Landy, Jimmy Harte and John Whelan were expected to be present and vote with the Government.
Had they been present, the vote would have been tied 27-27 and the Government would have defeated the motion with the casting vote of Seanad Cathaoirleach Paddy Burke.
Three of the other senators not present, Fianna Fáil's Jim Walsh and Fine Gael's Deirdre Clune and Tom Sheahan, had pre-arranged "pairs".
Two former members of the Fine Gael parliamentary party, Paul Bradford and Fidelma Healy Eames, voted with the Opposition.
A number of the Taoiseach's nominees, Jillian Van Turnhout, Fiach Mac Conghail, Mary Ann O'Brien, Marie Louise O'Donnell and Katherine Zappone, also voted with the Opposition.