One of the larger parades in the US, Rockland County in New York State, which normally draws between 40,000 and 65,000 people, went ahead despite a split over the presence of a convicted IRA bomber.
Mr Brian Pearson (50) was chosen to be Grand Marshal of the parade in Pearl River, 25 miles from New York. He was granted political asylum in the US in 1997 after spending 12 years in prison for blowing up two RUC police stations.
However, his presence prompted an unprecedented boycott of the parade by police and firefighters' organisations.
Mr Patrick Frawlyer, a member of the police honour guard which usually leads the parade but was absent this year, and president of the Orangetown Patrolman's Benevolent Association, told local reporters: "There were much more appropriate choices. This decision has stirred up strong emotions in our community during a sensitive time when we all need healing in light of recent events.
"This in no way reflects on the character of the individual selected, but rather on the organisation people may affiliate him with."
Mr Pearson, who works as a carpenter, defended his presence at the head of the parade.
"I think it's difficult to understand, coming from people who live in a free society, to think it's controversial for anyone to defend their own people and fight for freedom," he told the New York Journal News.
Meanwhile, in New York it was a bittersweet day, filled with joy, tears and memories as thousands took to the streets to celebrate the 241st annual parade.
At 12:30 p.m., as bagpipers played the last chords of God Bless America, marchers and spectators along Fifth Avenue faced south to where the World Trade Centre towers had stood and offered a moment of silence.
"It was an eerie feeling," said the President, Mrs McAleese, who attended the parade for the first time as President. "I can honestly say I don't think I've ever had a moment like that in my life before."
As New York fell silent, the only sound heard was the clacking of cables as flags brushed against poles.
"Many of those who were lost on September 11th would otherwise be marching proudly . . . We know their spirit marches with us," said Mrs McAleese.
The sight of 343 US flags carried by 343 firefighters to represent the members of that department killed in the twin towers drew roars from the crowd.
More than 1,000 firefighters marched, taking 10 minutes to pass the reviewing stand. Some 130 members of Dublin's Fire Brigade were also in attendance.
In addition to the Irish fire fighters and police killed on September 11th, a number of Irish workers in the financial sector perished.
"It's a little bit of a bittersweet celebration," the city's new mayor, Mr Mike Bloomberg, said at a Gracie Mansion breakfast before the parade.
"We lost so many people and many of them came from Ireland or their families did. But what we have to do is go forward and look to try to build a better world for their children. So it is a time to celebrate," Mr Bloomberg said.
At a Mass at St Patrick's Cathedral, Edward Cardinal Edward Egan, the Grand Marshal of the parade, said: "This St Patrick's Day we are celebrating, but we are celebrating, I believe we all understand, in an atmosphere of hurt."
President Bush attended the parade in Chicago, and several other cities held celebrations.