Agriculture historically looms large in Irish politics
The influential Irish Farmers' Association (IFA) has thrown its support behind the EU's Lisbon Treaty ahead of a key referendum on 12 June.
Ireland is the only one of the 27 EU member countries to try to ratify the treaty through a referendum rather than a parliamentary vote.
The IFA's decision comes as opinion polls show the No camp gaining ground on the pro-treaty groups.
The Irish PM has pledged to veto any trade deal that harms Irish farming.
In a statement on Tuesday the IFA's president Padraig Walshe said: "It is a unanimous agreement of the [IFA executive] council that we are prepared to support a 'Yes' vote and we will be writing to all our members to explain the situation to them".
Correspondents say the IFA has strong influence in rural areas, where many people depend on farming. Some of Ireland's top companies are food producers.
The Lisbon Treaty replaces the ill-fated EU constitution, which was rejected by French and Dutch voters in 2005.
A No vote in the referendum would dash hopes for an end to years of diplomatic wrangling over the EU's structures.
Ireland's Prime Minister, Brian Cowen, wants the EU to toughen its stance in the forthcoming world trade talks to ensure that Irish agriculture is not undermined by a flood of cheap food imports.