All the main political parties are urging a Yes vote, while the Catholic Church and many social conservatives want a No.
The fictional Mrs Brown, Ireland's mammy-in-chief, has made a video calling for a Yes vote in the referendum.
In it, Brendan O'Carroll's character tells the audience that when she "was a young girl there was a big hoo-ha about mixed marriages, Catholics marrying Protestants and black people marrying white people.
"But, you know what, they still went and got married and the world didn't end. And we all grew up a little bit."
Keith Mills doesn't agree with the video's core message.
He is a gay man opposed to same-sex marriage.
"I've had a relationship with a woman and had that progressed I would be a father and have children," he said.
"I've had relationships with men in the last few years since I came out and it's a totally different dynamic to the relationship.
"We're being asked to say they are the same and they are not. We should provide equality for same-sex couples through civil partnerships, but preserve marriage for a mother, a father and their children.
Back home after a day campaigning for a Yes vote, Denise Charlton and her civil partner Paula Fagan catch up.
A lesbian couple with two young boys they want to marry and say "I do" to each other.
Denise said it's all about equality and invokes the civil rights struggle of African Americans.
"Rosa Parks was allowed on the back of the bus. She could travel from A to B on the same bus.
"But there was an outcry about it because it was lesser and people recognised that," she said.
"I think Irish people increasingly are recognising that it is discriminatory not to allow people to have access to this wonderful institution. And giving an alternative institution with less rights is not equality."
Paula agreed and said a Yes vote would make it easier for young gay people to come out and be accepted and make families like theirs more complete to their young sons.
"They love us and they think our family is great.
"Our nine-year-old has said to us 'Everybody is voting Yes' because everyone in his class has told him their parents are voting Yes," she said.
"I think it would be devastating for them if there was a No vote because they love their family."
No campaigners have also taken to social media to get their message across.
One of their videos says: "If we vote Yes on 22 May we will be forced to pretend that two fathers or two mothers are just the same as a father and a mother.
"This is why marriage equality is really inequality for children."
All of the main political parties disagree with that and are urging a Yes vote.
But Senator RÃ³nÃ¡n Mullen is one of the few parliamentarians to say he's a No and he believes that he's part of a silent majority - shades of shy Tories.
"It may be that there are many Irish people who feel like they're being judged, that they're thought to be homophobic or intolerant simply because they want to vote in favour of a child's right to a father and a mother, but not against gay people and not against civil partnership, which is already there," he said.
"Those people will be sovereign at the ballot box and I hope there will be enough of them to reject this very far-reaching proposal."
Homosexual acts were only decriminalised in 1993, so to introduce same-sex marriage in a popular vote would constitute a massive change.
But with highly visible gay politicians, government ministers past and present, judges, sportsmen and women, business people and celebrities, the Republic of Ireland has changed a lot.
And it may seem odd that while gay people were the ones who were, historically, discriminated against and often violently beaten, it is those opposed to same-sex marriage who now feel they are victims of what they claim is a biased media and political correctness.
UCD professor of politics David Farrell believes modernisation and the Catholic church's many child sex abuse scandals and its diminished public standing, are a factor in the changes.
"I love to show my students two photographs juxtaposing Eamon De Valera kissing the ring of a bishop when he was president of Ireland with one of our current Taoiseach [prime minister] Enda Kenny, in a gay bar," he said.
"That's a huge path we've come. I never thought that could be possible in this country."
Whatever the polls may say both sides expect a close result and getting their vote out will be crucial.
Dublin and the more urban areas are believed to be more liberal and older people more conservative.
The result should be known some time on Saturday.
On Friday, voters in the Republic of Ireland could make history by introducing same-sex marriage by the ballot box rather than by a parliamentary vote or a judge's decision.