Do our politicians actually know what they are passing into law when they cast their votes in Leinster House? That is the worrying question thrown up by an academic researcher's poll of parliamentarians to ascertain their level of understanding when they passed a specific Oireachtas bill.

Of 72 TDs and senators who were asked if they were aware that the Civil Registration Act, 2004 contained a certain clause, just one replied that he did.

Take a bow Timmy Dooley, Fianna Fáil's poll-topper in Clare, the solitary parliamentarian to assert that he knew what he was voting for when section 2.2(e) of the bill went through both the Dáil and the Seanad without a debate. The clause stipulates that, for the purpose of the Act, "there is an impediment to a marriage if both parties are of the same sex".

In May and June this year, Cork-based researcher Jackie Mullins canvassed 28 senators and 44 TDs who voted for the bill five years ago and who are still Oireachtas members. She asked them if they were aware of the ban when they voted for it.

Three senators and nine TDs replied. Only one, Dooley, who was a senator at the time, claimed he knew about the prohibition on same-sex relationships in the bill. One of his party colleagues in the Seanad, whom Mullins is not prepared to identify, answered that she was unaware of the ban at the time she voted for it.

As the Dáil's €100,000-a-year deputies begin their two-months' summer holidays this weekend, the revelation that they do not even read the laws they make will compound their bad press. Before last year's Lisbon referendum, the taoiseach admitted he had not read the proposed treaty. His admission was followed by Brian Lenihan's confession that he did not read "the entirety" of the government-commissioned report by Price­waterhouseCoopers on the banking sector.

Most of the politicians polled about the Civil Registration Act were members of Fianna Fáil as Fine Gael and Labour deputies abstained from the Dáil vote.

An attempt by Green and Sinn Féin TDs to remove the clause by an amendment to the legislation was not debated and was defeated.

Despite making full legal recognition of gay relationships a precondition for entering coalition, the Greens acceeded to the watered-down commitment contained in the Civil Partnership Bill.

"I find this absolutely astonishing," said Mullins. "While I don't expect each representative to know every single line of any impending legislation, I do expect them to have a basic understanding of all of the important dynamics to it. The impact of that lack of knowledge goes far beyond the enactment of one piece of legislation.

"I think that it has huge implications for our parliamentary democracy, but my main concern is that this prohibitive and discriminatory clause is on the statute books and that many of our legislators haven't given it a moment's thought."