(Reuters) - The atmosphere is "buzzing" at the sellout Irish Open at Royal Portrush and the tournament has the feel of a major championship, world number 11 Graeme McDowell said on Tuesday.

The appearance of Northern Irish trio McDowell, Rory McIlroy and British Open winner Darren Clarke at this week's event, plus triple major champion Padraig Harrington of Ireland, has helped the European Tour to achieve a first by declaring a sellout for all four days.

Organizers are expecting around 100,000 people to turn up as the tournament returns to the famous old links course for the first time since 1947.

"When these crowds come in over the next few days it's going to feel like a major championship," McDowell told the BBC at County Antrim.

"The golf course will certainly be major championship standard and there's no doubt there will be a little bit of added expectation from the fans looking to see the Irish boys play well," added the 2010 U.S. Open winner.

"Royal Portrush is a golf club with a huge amount of history, given the British Open was staged here in 1951. But in the modern era this is one of the proudest moments for this golf club, for the area and for Northern Ireland.

"It's a huge thrill to see such a great field assembled and everyone is buzzing and excited."

World number two McIlroy has fond memories of Portrush where as a 16-year-old he shot a record 11-under 61 during the 2005 North of Ireland Championship.

Now 23, this will be the first time 2011 U.S. Open winner McIlroy has played in front of his home fans as a professional.

"To be one of the three or four guys who helped make this happen through our major successes also gives me a huge sense of pride - it's great to have played a part and it's kind of a dream come true," added McDowell.

The Irish Open starts on Thursday.

(Writing by Mark Pangallo; editing by Tony Jimenez)