The Dáil has been warned that lessons need to be learned to avoid repeating the same mistakes in the referendum on marriage equality that were made in the vote on the abolition of the Seanad.

Language used in the Seanad referendum was not plain English, but “gobbledygook” and might have contributed to the high number of spoilt votes.

It was even worse in Irish, said Sinn Féin’s Aengus Ó Snodaigh, because the language used was not everyday speech. Mr Ó Snodaigh is a fluent Irish speaker, but the question on referendum papers was framed “in such legalistic terms that people could not make head nor tail of it, unless they had a copy of the Constitution” in front of them.

The Dublin South Central TD said: “I hope we will have learned the lessons from the Seanad referendum by the spring,” when the same-sex marriage vote is due. “The proposition to be put must be made very clear in order that the public will be able to make an informed decision.”

He was speaking during a debate on the report by the Oireachtas public oversight committee on the Seanad referendum in October last year, following a number of complaints to the committee about the ballot paper. Voting Yes, Mr Ó Snodaigh said, meant voting to abolish the Seanad “and was not an affirmation to support it”.

He also called for larger print to be used. “Recently the print used has been smaller clearly because it is costlier to produce more pages.” He said that “if it means an extra €1 million to produce documentation that is clear and legible, so be it”.

Minister of State Kevin Humphreys agreed there was a need for clarity of language on the ballot paper and clarity between English and Irish. He said the ballot paper for referendums had remained largely unchanged since 1963, while there had been significant changes to the ballot papers for general elections.

However, he said, research showed the number of spoilt votes in referendums rose “when the electorate are asked to vote on more than one issue on the same day”. A second referendum on the establishment of the Court of Appeal was held on the same day as the Seanad abolition referendum.

The president of the oversight committee, Sinn Féin’s Pádraig Mac Lochlainn, said multiple referendums on the same day might have been an issue, along with the design, text and language of the ballot paper and the phrasing of the question, which, he said, was effectively “a double negative”.