Wedding businesses are preparing for a surge in business if the referendum is passed next Friday.
Hotels, wedding photographers, bridal shops and other companies can expect hundreds of extra marriages if the amendment to the constitution is carried.
Mark Downing set up civilpartnerships.ie after civil partnerships for gay and lesbian couples were legalised in 2011. His company books entertainment for gay and lesbian couples.
"There has been 100pc growth in this business every year and we can expect a dramatic increase in bookings if the referendum is carried," says Mr Downing.
He looks forward to rebranding his business to take account of the change in the law.
Marian Purcell, who set up Gay Weddings Ireland with her partner Shane Costelloe, is looking forward to a similar surge in business if the vote is carried.
Her Limerick-based website offers links to "gay-friendly" wedding businesses, including photographers, bands, cake suppliers, clothing shops, stationers and florists.
She is already organising the Gay Wedding Show at the Westin Hotel in Dublin on June 26.
Marian Purcell says gay couples often spend a lot on weddings, because on average they tend to have a higher disposable income.
Owners of bridal shops, in particular, are looking forward to a boom. With two brides at lesbian nuptials they can double their money. The same goes for shops selling grooms' attire.
So far, over 3,000 gay and lesbian people have entered into civil partnerships in Ireland, and hundreds of others have got married abroad.
"It tends to have been older couples who got civil partnerships," says Marian Purcell. "A lot of younger people have held off, and are waiting to see what happens. I expect many of them will get married when it becomes legal."