The website of Christian pro-life group Youth Defence has been hacked to display a pro-gay marriage message.
The website banner now reads 'Vote Yes', and carries the image of a pro-Yes mural on George's Street in Dublin.
The Dublin mural, called "Our Nation's Sons", was created by street artist Joe Caslin.
It shows two men hugging in a promotion of gay rights.
Other areas of the site appear to be inaccessible.
This latest incident comes just a day after another pro-gay marriage mural - depicting two women - was defaced on Dublin's South Richmond Street.