Some 2,000 protestors gathered near the Ashtown Gate of the Phoenix Park. As tension increased gardaí in riot gear were brought in and water cannon were deployed.
The gardaí began pushing the protesters backwards in the direction of the city.
One female garda was injured and 40 protesters were arrested. They were said to be from London and Cardiff. Gardaí on horseback were later brought in as the situation eased off.
Over 600 protestors taking part in a march organised by the Dublin Grassroots Network moved off in the direction of the Pheonix Park earlier.
It was reported that a decision had been made to march on Farmleigh, where EU leaders are enjoying a banquet, following a rally outside the GPO.
It followed three and a half hours of a Reclaim The Streets protest which included a demonstration outside the Department of Justice and the occupation of Fitzwilliam Square Park.
Meanwhile, up to 1,000 protestors from an anti-war and anti-racism group, Another Europe is Possible, gathered near Heuston Station for a carnival.
The march passed off peacefully. The protestors gathered on the south side of the Liffey by the train station but were not allowed cross over the river to the park itself.
European market at Merrion Square
A daylong 'European Fair' took place on Merrion Square. The square was lined with marquees from the EU's 15 existing member states, the ten new members and the three pre- accession countries, with free food, drink and music.
Merrion Square is closed to traffic until tomorrow morning. The Phoenix Park is closed to all traffic, and there are restrictions on roads around the park.
Last night, thousands of people gathered at a large fireworks event at Sandymount beach in south Dublin to mark the historic occasion.