In a last-minute effort to end a feud among Europe's wine producers that is blocking the entry of Spain and Portugal into the European Community, Irish Prime Minister Garret FitzGerald presented a new plan tonight to European leaders at the opening of a two-day summit meeting.

Aides to FitzGerald, who is the current president of the European Community, said the proposal -- a complicated plan for sharing the burden of cutbacks in wine production initially submitted behind closed doors -- was well-received. Some cautious optimism was expressed as the talks shifted to lower levels to try to work out details.

Failure of the Europeans to resolve the feud -- which is primarily among France, Italy and West Germany, and involves efforts to curb the European community's excessive wine production -- would further jeopardize chances for meeting the Jan. 1, 1986, deadline for expanding the EC membership to 12 nations.

At stake may be the political future of Spain's socialist Prime Minister Felipe Gonzales, who has indirectly linked Spain's admittance to the EC with his country's continued membership in NATO.

Gonzales has scheduled a referendum on NATO membership in February 1986, one month after Spain is supposed to be admitted to the EC. If admission is delayed or rebuffed, however, this could feed the substantial opposition in Spain to continued NATO membership and undermine the Gonzales government, which favors staying in NATO.

There is considerable pressure within the EC and from the United States to settle the disputes and get Spain and Portugal -- both relatively new democracies -- admitted to the community. Negotiations on the issue have been under way for almost seven years.

EC wine producers already turn out a surplus of wine that costs about $750 million a year in subsidies for production and storage.

France and Italy would suffer most by the entry of Spain and Portugal, which also are big wine producers, and want the issue solved before the agreement on entry is concluded.

Spain's huge fishing fleet is another problem. But if the wine issue is settled, it is believed the other issues will be resolved, diplomats said.

A failed summit here could undermine the already tarnished image of European unity at a time when other controversial proposals are being aired.

The heads of governments were presented today with an interim report of a committee that could foreshadow sweeping changes in the community.

The committee, headed by former Irish foreign minister James Dooge, was set up at the last summit in France in June. Scheduled to give its final report next March, it has called for "a qualitative leap" by Europe to achieve greater political will and unity if it is ever to rectify Europe's poor economic and job-creation performance in comparison to the United States and Japan.

Some interim proposals include restricting the current right of any member state to veto decisions, a much greater reliance on majority rule, increasing the legislative powers of the European Parliament and replacing the current Treaty of Rome, which set up the EC, with a new Treaty of European Union.

British Prime Minister Margaret Thatcher has made clear that while she supports greater cooperation, she feels that Europe should first end the kind of protectionism and restraints on trade in such areas as excessively high intra-European air fares and border bottlenecks for truck traffic.

Thatcher's presence here, almost two months after an IRA bomb in England almost killed her, brought out perhaps the tightest security in the history of EC summits.

About 2,000 Irish police sealed off streets for blocks around the 13th century Dublin Castle,the site for this two-day summit.