Facebook users had an accidental glimpse of the international scope of campaigning in Ireland’s historic abortion referendum – set for 25 May – which has intensified concern about foreign influence on the vote.

For a brief period last Friday, a new tool designed to boost transparency around elections enabled users to see the real-life location of people who were managing Facebook campaign pages. Activists collected a snapshot of posts from 180 groups that were targeting Irish voters, before the social media giant turned off the location tool, saying it had been “mistakenly launched”.

This evidence showed that a significant proportion of posts aimed at influencing voters came from pages managed partly or entirely outside Ireland. Even some groups registered with Ireland’s ethics watchdog, the Standards In Public Office Commission (Sipo), had Facebook pages managed from abroad.

Foreign influence seemed particularly strong in messages urging Irish voters not to overturn the country’s near-total ban on terminating pregnancies. Ireland is the only western democracy with such strict rules, and the upcoming vote has drawn worldwide attention from opponents of abortion rights. The referendum will ask voters whether they want to repeal article 40.3.3 – known as the eighth amendment – which enshrined a ban on abortion in the Irish constitution in 1983.

The Yes campaign is in the lead, but polls have been tightening recently, and the 20% of Irish voters who say they are still undecided are thought to hold the key to the referendum result.

Analysis of the posts, by social media news agency Storyful, found that only a third of advertisements urging a No vote – which would preserve the strict abortion law – came from Facebook pages managed solely in Ireland. In contrast, four-fifths of posts urging repeal of the amendment came from pages which had no foreign-based managers.

The anti-abortion pages that had at least some foreign managers included those of some influential groups campaigning for Ireland to keep its abortion ban. One had listed managers, of whom some were based in the UK, some in Hungary and others in unidentified countries.

The data also showed that posts supporting a No vote were more likely to have come from organisations that were not registered with Ireland’s ethics watchdog.

Facebook did not give details of how it identifies location, so some of the managers listed as overseas could be Irish citizens on a temporary trip, or who had previously lived abroad, or used virtual private networks (VPNs) that made them appear to be out of the country.

Demonstrators at a rally this month opposing the repeal of Ireland’s near-total ban on pregnancy terminations. Photograph: Clodagh Kilcoyne/Reuters

The brief revelation gave further insights into how much important data Facebook holds – but has chosen to keep secret – about election campaigning and advertising. These are areas that all democracies have traditionally monitored to ensure that votes are free and fair.

“This is another window into some of the information [social media] platforms hold, and it’s the kind of information that should be made available,” said Liz Carolan of the Transparent Referendum Initiative, which created the database of ads. “Let us know what’s going on, then we can regulate this as a country.”

Her group does not take a position on the referendum result, but wants to make sure that voters have a clear vision of who is campaigning and how they are paying for it.

They had earlier exposed the role of foreign groups in paying for social media ads that were visible only to the target group – using a legal loophole to join the campaign from perhaps thousands of miles away. Irish law prohibits political donations by foreign nationals, but the legislation does not cover digital advertising, which meant foreigners could fund ads from the first week of campaigning.

One group that paid for ads aimed at Irish voters was US group EMC FrontLine Pregnancy Centers, which claims on its website to “rescue moms and babies”. In January it was fined by New York City authorities for misleading women about their healthcare options. Another is Toronto-based LifeSiteNews, whose headlines include: “Social media uses #fakenews and Stalinist efforts to help repeal Ireland’s pro-life 8th amendment”. Neither group responded to the Observer’s requests for information.

Concerns about foreign influence have prompted Google to ban all online advertising connected to the referendum, and Facebook to limit ad sales to organisations based in Ireland. Facebook also promised to let users see all political ads running on the site in real time.

Google’s ban in particular has been attacked by anti-abortion groups, who insist they face bias from traditional media and need social media to combat it. David Quinn of Catholic thinktank the Iona institute tweeted last week: “Of course mainstream media dislike social media advertising by pro-lifers. Such ads are a way of doing an end-run around media bias.Google are worried that the Yes side would blame them if they lose [the referendum]. Obviously they couldn’t care less if it has the opposite effect. That alone shows terrible bias.”