Scuffles broke out in central Dublin at the launch of the anti-Islamisation group Pegida.
Hundreds of campaigners, from a number of grassroots groups, came out in protest.
Dublin City Councillor Ciaran Cuffe of the Green Party tweeted of a "strong security presence" and some protesters pursued several men they believed to be Pegida members from O'Connell Street down onto North Earl Street.
The streets were particularly busy. Worried families, including children who were in the city for a Chinese New Year festival, and sports fans who had arrived for the Ireland and Wales RBS 6 Nations rugby match, saw the commotion unfold.
Pegida's Dublin launch was among a series of Europe-wide demonstrations against the growth of Islam in Europe.
Banners declaring Love Not Hate and No To Islamophobia No To Pegida were held by protesters who came from different backgrounds.
Organisations such as People Before Profit, the Workers Solidarity Movement and Irish Muslim organisations were among the demonstrators.
Sinn Fein MEP for Dublin Lynn Boylan told an anti-racism rally on O'Connell Street: " We are standing shoulder to shoulder in solidarity to show that there is no place in Ireland for racism and Islamophobia.
"There is no place in Ireland for hate.
"We are a welcoming nation because we are no strangers to migration."