The proposal to amend the Irish constitution to include same-sex marriage right is expected to pass, as the Yes side has held a double-digit lead in opinion polls during the two-month campaign.
Voting began at 7 a.m. local time and continues until 10 p.m. The results will be announced on Saturday.
As with previous Irish votes, such as joining the European Union, banning abortion and legalizing divorce, the issue of same-sex marriage legality is drawing people to the polls. Electoral officers reported stronger-than-usual turnout at many stations in schools, church halls and pubs across the nation of 3.2 million registered voters.
On the streets of Dublin, Liz McDermott, a member of a group called Mothers and Fathers Matter, spent hours debating Yes side supporters while she canvassed for the No side.
"I do recognize this is a kind of old-fashioned perspective on things," McDermott told CBC News correspondent Nahlah Ayed.
"It's my democratic right to be here and to speak my mind. I feel actually more than that. It's not my democratic right, it's actually my duty," she said.
"Parenthood is so important. It's so important we get it right. It's so important we take marriage seriously."
Irish Senator Katherine Zappone is one of the most prominent Yes supporters. She got married to her same-sex spouse in Vancouver in 2003. That marriage will be recognized as legal in Ireland if the Yes side wins," she said.
"It's like we were outcasts, exiled," Zappone told the CBC's Ayed. "We had to go outside the country in order to access what we considered to be our fundamental human right."
"The majority will demonstrate what I believe is at the heart of what it means to be Irish: their fairness, their compassion, their generosity," she said. "But they are freedom fighters! We are coming up [in 2016] to 100 years of celebrating the birth of our republic... and that's so much what it means to be Irish."