Retailers across Galway city will not sell alcohol before 3pm today.
Off licences and supermarkets have entered a voluntary agreement with the Gardaí for St Patrick's Day.
It is hoped the move will prevent anti-social behaviour on the city's streets.
Lord Mayor of Galway City, Councillor Frank Fahy, said public drunkenness should not be tolerated.
He said: "It's a brilliant idea and I commend all the shop owners and off-licences for doing this.
"It shows great public spirit and it should help to prevent public drunkenness.
"We have an issue - not just in Galway. We sort of lose the plot and go all-out on drink. It should not be tolerated."
Meanwhile, more than 100,000 tourists have come to Ireland for the festivities, with half a million expected to line the streets of Dublin for the largest parade in the country.
A larger than normal garda presence will be seen in the capital to clamp down on anti-social behaviour and to help deal with the crowds.