A group against same-sex marriage has vowed not to let Australia go the same way as Ireland and "spit on something they once held sacred".
But gay activists say the usually conservative country's strong approval of same-sex marriage will have repercussions in Australia.
Ireland became the first country in the world to approve gay marriage by popular vote in a referendum on Saturday, with an overwhelming 62 per cent voting "Yes".
The result has spurred a strong response from the Australian Marriage Forum, which has previously run controversial ads against marriage reforms.
"This is a day of shame for our beloved Ireland," the group's president David van Gend said.
"If we are the last country standing, we will not abandon children to a motherless or fatherless future just to gratify the demands of homosexual adults."
Australian Marriage Equality director Rodney Croome says the strong result "quashes the myth" that religious and rural people are opposed to reform.
It will leave Australians "scratching their heads" over why the federal parliament has not supported reforms when they are being approved in more socially conservative countries, he said.
"The scale of support will magnify the embarrassment that many Australians feel about our country being left behind," he told AAP.
A referendum in Ireland was required because marriage is defined as between a man and a woman in the country's constitution.
It could be achieved through legislation in Australia.
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