DUBLIN: The referendum has divided voters in a traditionally Roman Catholic country where sign of changing attitudes have emerged in recent years. Here's what to look out for:
WHAT WILL BE DECIDED?
A "yes" or "no" vote will decide whether Ireland repeals or keeps the controversial eighth amendment to the constitution, which in 1983 required authorities to equally defend the right to life of a mother and the right to life of a fetus, from the moment of conception.
Abortion is only permissible in very rare cases when the woman's life is judged to be in danger.
WHAT ARE THE ARGUMENTS?
Opponents say the law simply forces women to leave their home country at considerable expense to get safe abortions elsewhere. They also cite cases where women have been placed in danger because doctors were unsure whether they could legally carry out an abortion despite serious medical issues facing the pregnant woman.
Supporters of the eighth amendment say it has prevented Ireland from becoming an "abortion on demand" country like much of Europe. They cite the proliferation of abortions in Britain since the procedure was legalized there in 1967 as something they are determined to prevent. Many "no" posters and advertisements emphasize that a child's heart starts to beat just weeks after conception.
WHAT HAS BEEN THE AMENDMENT'S IMPACT?
Abortion was illegal in Ireland before 1983, but the amendment wrote the ban into the country's constitution. Women who get illegal abortions can face up to 14 years in prison.
Since a legal challenge in 1992, the law has allowed Irish women to travel to another country to obtain an abortion. Advocates of repeal say this can be expensive and sometimes traumatic, and amounts to outsourcing the issue to Britain.
WHICH SIDE WILL PREVAIL?
Many expect the results to be close. Ireland has strong Roman Catholic roots and opposition to abortion runs deep here, but the surprising results of a 2015 referendum that legalized same-sex marriage may indicate a more liberal view.
The country has its first openly gay prime minister, who is also the first from an ethnic minority group, which may suggest a more flexible approach to social issues.
WHAT HAPPENS IF 'YES' WINS?
If the "yes" side prevails, it would be up to parliament to come up with new laws on abortion.
The Irish government, headed by "yes" backer Prime Minister Leo Varadkar, has proposed legislation that would allow the termination of pregnancies within the first 12 weeks.
It would also allow later abortions if doctors believe there is a risk of death or serious harm to the pregnant woman. Doctors would also be allowed to abort if the fetus is determined to have a defect that would lead to its death in the womb or shortly after birth. These proposals are likely to provoke intense debates in parliament and it isn't clear what law would ultimately be put in place.
WHO CAN TAKE PART?
Irish citizens must be 18 or older to vote in the referendum.
"Yes" backers are hoping that thousands of young Irish men and women working or studying in Europe will come back and take part if they meet the legal requirements.
The "no" side is banking on a strong vote in rural areas and smaller cities to offset what is likely to be strong support for repeal in the capital, Dublin.
More than 3.1 million people are eligible to vote. Most polling stations will be open from 7 a.m. until 10 p.m. Friday, although voting has already begun on some remote islands. Results are expected Saturday.
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