The Government is under pressure to give State recognition to unofficial custody agreements between unmarried and separated parents following a rare Seanad victory for the Opposition over the Government.
The amendment to the Civil Registration Bill, moved by Dublin-based Fine Gael Senator Brian Hayes, was first defeated in an electronic vote by the Government.
However, Mr Hayes immediately demanded a manual vote, which the Government lost by 24-23 when two Fianna Fáil senators failed to get into the chamber in time to cast their ballot.
He said: "This is not about the vote. There are thousands of private arrangements in place where parents agree custody, access and other arrangements that are not registered with anyone.
"Relations between the parents can break down later on.
"There can be difficulties getting exam results, problems when one of the parents wants to go abroad with a child.
"If the informal agreements were registered people could then have something that they could go into court with and say, 'this was agreed'," said Mr Hayes.
He said he had been heavily lobbied by a number of fathers' groups, who believe they are unfairly losing access to their children: "This is about legitimacy," Mr Hayes declared.
The Seanad defeat led to recriminations between the Government and the Opposition last night, amid charges that two Fine Gael senators had voted even though they were said to have paired.
However, Mr Hayes told The Irish Times: "That is not true. We had no pairing arrangements in place for the 5 o'clock vote. There were arrangements for votes at 7 p.m., but not before."
The Minister for Social and Family Affairs, Ms Coughlan, is determined, however, that the amendment will be deleted when the Bill comes back to the Seanad for its final stages.
The current legislation is designed only to update existing registers and not create new ones: "If a guardianship register was to be created it would come under the Courts Service," said her spokeswoman last night.
