5 Things to Know: Key questions and answers on Ireland's vote on legalizing gay marriage

DUBLIN — Ireland's voters decide Friday whether to legalize gay marriage. While 19 other nations and most U.S. states have already done so, Ireland is the first to hold a national vote. Though voting is Friday, results won't be announced until Saturday.

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WHY IS IRELAND VOTING?

Ireland's 1937 constitution, written by then-Prime Minister Eamon De Valera in consultation with Catholic Church leaders, is a document laden with conservative Christian values. It affords special legal protections to married couples, committing state institutions "to guard with special care the institution of marriage, on which the family is founded, and to protect it against attack."

Proposed laws deemed at potential odds with that landmark document must be added to the constitution by popular vote. A single vote over 50 percent "yes" is sufficient for any constitutional amendment to pass.

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WHAT IS IT VOTING ON?

If this referendum is passed, a new passage will be added: "Marriage may be contracted in accordance with law by two persons without distinction as to their sex."

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WHO VOTES AND HOW?

Any Irish citizen over 18 is eligible and more than 3.2 million are on the electoral register, including 66,000 who barely beat the registration deadline, most of them young people voting for the first time. They'll be asked to mark a simple X on YES or NO, or TA or NIL in Gaelic, Ireland's little-used native tongue.

Ireland's voting system requires physical presence at a specific polling station. This is a problem in a country where many students and young professionals work in the capital, Dublin, but have countryside family homes on the far side of the island. Ireland's exceptional rates of emigration in response to its 2008-2013 debt crisis mean Friday's journey for some voters is even longer.

Thousands of emigrants are flying to Ireland to vote, mostly from neighboring Britain. Arriving flights from London and other British cities are virtually sold out for Friday, a rarity.

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WHICH SIDE WILL WIN?

Eight opinion polls in the past two months of campaigning have recorded a strong lead for "yes" voters. But Irish referendums often produce surprise results. Analysts say several factors could produce a louder-than-expected "no" when ballots are counted Saturday.

Referendums provide a lightning rod for anti-government sentiment, regardless of the issue at stake. All political parties and most politicians are backing the "yes" campaign. Anti-gay marriage campaigners, led by the Catholic Church and a conservative think tank called the Iona Institute, argue that the lopsided support from the political establishment should raise suspicions for ordinary voters.