With just five days to go before Ireland’s historic referendum on the legalisation of gay marriage, a bitter row has broken out between supporters and opponents over the funding of their respective campaigns. Supporters of a yes vote have accused opponents of a lack of transparency over finances and of accepting funding from rightwing Christian groups in the US.

Ireland’s referendum is the only one in the world where a national electorate is being asked to legalise gay marriage. If it is passed on Friday, gay couples will have the right under the state’s constitution to marry – at present only civil partnerships are recognised in law. Marriage equality would provide full rights of inheritance but, despite it being a contentious issue in the campaign, would not alter the law on adoption, which was recently changed to allow gay couples to adopt.

A yes vote would also mark yet another defeat for the Catholic church and the political power it used to wield in Ireland.

The yes campaign says its opponents have a huge advantage in terms of resources for buying billboard and poster space and have spent tens of thousands of euros in the last few weeks alone. Now no campaigners have booked full-page advertisements to appear later this week in dozens of regional Irish papers, calling on voters to reject incorporating the right for gay couples to marry into the republic’s constitution.

One of the no side’s strongest supporters in the US is the lavishly funded National Organisation for Marriage (NOM). In a letter to supporters around the world, it has urged evangelical Christians to visit keepmarriage.org, which is campaigning for a no vote.

“Just like in campaigns for marriage here in America,” the letter says, “slanted public opinion polls become fodder to influence and depress supporters of marriage. This is happening in Ireland. If [the no campaign] can manage to pull off a victory, it will be a tremendous boost to the cause of marriage worldwide. Please do what you can to bring awareness to their efforts.”

However, a spokesperson for NOM in the US denied this weekend that it had channelled funds to any of the three main opposition groups to gay marriage during the campaign. NOM is aware that foreign donations to lobby groups during referendums in Ireland are banned, the spokesperson said.

Some in the no campaign have countered with claims that the yes side has benefited from millions of dollars donated by the Irish-American multimillionaire Chuck Feeney and his Atlantic Philanthropies agency.

John Waters, the award-winning Irish columnist and writer, heads the First Families First group, which opposes gay marriage. Waters said it was outrageous that Feeney’s agency had been “allowed to swamp the Irish democratic process”. He and independent Irish senator Rónán Mullen allege that Atlantic Philanthropies is financially backing the yes campaign because it funds a series of pro-gay marriage organisations ranging from the Gay and Lesbian Network to Amnesty International in Ireland.

Asked who funds his own group, Waters said: “We have no funding whatsoever and are not seeking any, being simply three private citizens seeking to defend certain constitutional rights and protections.”

Atlantic Philanthropies declined to answer questions about the claims, but backers of the yes campaign firmly rejected them. They said their group adhered to the strict rules on campaign funding set up by Ireland’s Standards in Public Office (Sipo) commission. The Sipo register of lobby groups shows that at least 10 of the pro-gay marriage organisations have fully complied with its rules, including a ban on foreign donations.

Brian Sheehan, the co-director of pro-gay marriage group Yes Equality, said: “Atlantic Philanthropies are not funding the Yes Equality referendum campaign. Yes Equality is fully funded through its supporters organising fundraising initiatives throughout Ireland.

“In addition we ran a crowdfunding campaign to raise monies for our poster, bus tour and booklet campaigns. All elements of the Yes Equality campaign are appropriately registered with the Standards in Public Office commission. Yes Equality is entirely dependent on generous small donations from around the country. The average donation made to Yes Equality has been €70.”

Despite the accusations and counter-accusations over foreign money, the no camp appears to be facing an uphill struggle in its battle to defeat proposals to legalise gay marriage. A poll by Ipsos MRBI for the Irish Times this weekend found that 58% will vote yes while 25% will say no, with 17% of the 1,200 surveyed undecided.

And in a late bid to stop gay marriage become legal in a state where homosexuality was a crime until 1993, four Irish Catholic bishops released pastoral letters late on Saturday night urging their parishioners to vote no on 22 May. The statements, from Bishops Kieran O’Reilly, Kevin Doran, Martin Drennan and John Fleming will be read out in churches on Sunday. The pro-gay marriage cause also received a significant boost in this last weekend of campaigning when the Republic’s most famous country music star, Daniel O’Donnell, who has an enormous fanbase in rural, conservative Ireland, came out in favour of a yes vote.