College Green is pedestrianised until 7pm for the next three Sundays.
There are plans for family events on the three days, including a picnic and live music.
Cars travelling down Dame Street towards College Green will be diverted towards the South Quays and will be permitted to turn right up George's Street.
Dublin City Council is trying out the concept after An Bord Pleanála last October turned down a plan to permanently pedestrianise the area.
Dublin Town chief executive Richard Guiney says the practical demonstration may win over critics. I'm reminded [of] Janet Sadik-Khan when she was proposing to create a plaza on Times Square. People were saying there'd be traffic jams from Montréal to Mexico City if that went ahead."
"They said 'let's do it for a week' and it was a success," he said.
"We're a great believer in triall things like this -- it gives you the opportunity to see whether it works or doesn't work.
Green Party councillor for the South West Inner City Michael Pidgeon says the College Green needs to be enjoyed by everyone.
"We're saying if you're driving into Dublin, please continue to do so. Park up and come into College Green and enjoy the space. At the moment the street is only really for buses, taxis, cars and the odd Luas going through."
Yesterday a coalition of cycling groups and urban environmental activists shut down vehicle traffic on South William Street in Dublin 2 to create a "clean air zone".