"This is a once-in-a-generation vote," according to Ireland's premier, Leo Varadkar. In a debate that divides Ireland so deeply down the middle, it is one of the few statements everyone can agree on.

Turnout could be higher than for gay marriage in 2015 which was approved by 61 percent of voters if trends reported by national broadcaster RTE in the late afternoon were sustained into the evening. The polls close at 2100 UTC with the first exit polls expected shortly afterwards. The votes are to be counted on Saturday morning.

The near-total ban on abortion currently in place was adopted as the "Eight Amendment" to the Irish constitution. This clause, passed in a contentious referendum, aimed to stop Ireland's courts following the lead of other countries and introducing abortion without the approval of the legislature. But what was intended as a complete ban on termination has not turned out this way: a landmark case in 1992, involving a suicidal teenage rape victim known only as "X," found that abortion could be permitted in cases where the pregnancy poses a "real and substantial" risk to the mother's life.

Read more: Rubicon crossed in Ireland's abortion debate

Other landmark cases have emerged since then, but none have widened the net of abortion. Women seeking terminations in any other cases, such as following a rape or if their child will not survive outside the womb, cannot do so in Ireland and have to travel abroad. More recently, an asylum seeker hoping to travel for an abortion elsewhere was intercepted and detained until her pregnancy had reached viability and labor induced. In 2014, a brain-dead woman was kept on life support simply because she was 15 weeks pregnant; although there was no prospect of keeping the child alive until it was viable, doctors were unsure whether they were permitted to turn off the mother's life support if it meant ending the fetal life.

Savita Halappanvar, who under Irish law was not entitled to an abortion, died after complications contracted during a miscarriage

Read more: Ireland's abortion referendum: What you need to know

Legislation needed

These are the "hard cases" cited by "Yes" campaigners as grounds to remove this 1983 prohibition — in their own words, to "Repeal the Eighth." "The Eighth amendment has not worked," Ireland's health minister, Simon Harris, told DW. Harris sponsored the parliamentary legislation to bring about Friday's vote. "If its aim was to stop abortions, by any measure it has failed. Every day nine Irish women go to the UK for a termination, and at least three Irish women take abortion pills illegally without any medical supervision. You need to be able to legislate and put a legal and clinical framework around such sensitive issues."

This is why Harris and his colleagues are proposing to repeal the Eighth Amendment, replacing it with a provision removing the constraints on parliament, so that lawmakers can pass whatever abortion law they see fit. Harris's proposals would include allowing abortion in cases of "fatal fetal abnormality," as well as where pregnancy poses a serious risk to a woman's health.

A more contentious proposal, however, is a plan to allow unrestricted abortion for the first 12 weeks of pregnancy. This is aimed to cater to victims of rape, but also opens the door to what anti-referendum campaigners have called "social abortions," and the prospect of ending a pregnancy over fears of a non-fatal disability.

"All the Eighth Amendment does, actually, is prevent a doctor from terminating the life of the unborn baby — and that is all we're voting about on Friday," said Caroline Simons, a legal adviser to the "Love Both" anti-abortion campaign. Speaking at a press conference this week, Simons told reporters: "There is absolutely not one shred of evidence that it is necessary to avert a health complication of pregnancy, or a life threat to a woman in pregnancy, to directly and intentionally kill your baby."

Influence from abroad?

While the 2018 debate has not repeated the caustic tone of the 1983 original, it has nonetheless reopened some old scars. Almost every section of society is divided; every time a leading doctor or lawyer declares support for "Yes," another will emerge to lobby for "No."

Though only Irish citizens have a vote, the sensitive nature of abortion has inevitably attracted foreign attention, and each side accuses the other of befitting from foreign funding.

Meanwhile, local religious-backed groups have harnessed the help of US-based campaigners to further their reach online, where there are no legal restrictions on advertising. One website, anonymously set up to offer information to undecided voters, was found to incorporate a Facebook tracking pixel which could then be used to identify undecided voters. The page's owner — later found to be a religious group which had recruited American help for its campaign — could then target those undecided voters with intensive Vote No arguments in the final days.

With the echoes of Brexit and Trump ringing in their ears, major online platforms were perhaps wary of being connected to further allegations of foreign voter manipulation. Facebook stopped all referendum advertising to Irish voters from foreign users. Google went one step further, banning all referendum advertising — including pre-roll ads on YouTube, a favored (and legal) means of the Vote No lobby.

For anti-abortion campaigners, this was a step too far. The major groups campaigning for a No vote issued a joint statement, labeling Google's decision as "an attempt to rig the referendum. Online was the only platform available to the No campaign to speak to voters directly. That platform is now being undermined, in order to prevent the public from hearing the message of one side."

James Lawless, a lawmaker with the opposition Fianna Fáil party, disagrees. He points out that Ireland's electoral laws received their last overhaul in 1992, in the days before online campaigning. "The fact that Google and Facebook took their own decisions to curtail advertising took an unregulated space out of the campaign," he told DW. "It would be preferable to have that regulated, but better to have it offline in a sense, rather than the Wild West we would have otherwise."

Doubts over future laws

Another odd factor of the referendum has been the confusion and disagreement about what comes next. While the ruling administration has published a draft law outlining the circumstances in which abortion would be permitted, it is merely a draft. Not only does this draft need to be turned into comprehensive law, but Leo Varadkar's government controls barely a third of votes in parliament, and has guaranteed MPs a "free vote" whenever the law finally comes before them. The second-largest group, Fianna Fáil, has taken a similar position — while Sinn Féin must hold a special conference before agreeing its position.

All of which means Friday's decision could be a momentous one for Ireland, and perhaps mark the end of a decades-long struggle for women's rights — or further entrench one of the western world's most restrictive regimes. But even if it passes, the future of Ireland's abortion law will be far from certain.