Among the protesters marching on a road in Athlone in central Ireland on Saturday were tractors and placards reading "Stop EU-Mercosur."
The protests follow similar initiatives in Poland and France, where farmers blocked roads on Friday after the agreement was given the green light via a written procedure in which a majority of the Union's member states voted yes.
The EU and the Mercosur countries – Brazil, Uruguay, Argentina and Paraguay – have been negotiating a free trade agreement since 1999.
EU farmers have long resisted the agreement, arguing that prices for agricultural products could fall if cheaper goods from Latin America enter the European market.
