They are like brothers, and some of them are. They spend their working hours together in the stations or putting out fires in the high rise buildings of New York, constantly dependant on each other.
When they are off duty, they socialise with their families at football and baseball games, barbecues and community events. They are godparents to each other's children, they attend anniversary parties and weddings together. And now, funerals.
New York city has 11,500 firefighters and almost 350 of them perished in the aftermath of the attacks on the World Trade Centre. The extent of the catastrophe for the fire service is shown by the figures. Since it was established 150 years ago, 700 firefighters have lost their lives on duty.
The attacks on the trade centre meant that almost half that number died in one day. Entire rescue units and specialised squads were wiped out as the call to the towers came just as one shift was finishing and another starting.
A photographer captured the image of a fireman rushing up the stairs as hundreds streamed down the same steps of the first tower, just before it collapsed.
The front page of the New York Post gave a chilling roll call of the 403 firefighters, police officers and port authority police who died when the towers imploded. Almost a third of the names were Irish - Brian Ahearn, Michael Boyle, Kevin Bracken, Michael Brennan, John Collins, John Heffernan, Thomas O'Hagan, Raymond Murphy and Richard Muldowney - just some of the names from the Manhattan fire stations which were most affected.
There were firefighters too from other New York boroughs: Eugene Whelan, Patrick Byrne from Brooklyn stations; Patrick O'Keefe, John Bergin and Peter Brennan of Staten Island.
The Irish-American toll is extensive. New York City fire department is an Irish institution. Of the 11,500 firefighters in the city, almost 5,000 of them are Irish or of Irish extraction.
Mr Bill Whelan, a native of Tullamore, Co Offaly, is president of the Fire Department of New York Emerald Society, a fraternal organisation for the Irish firemen in New York.
Of the devastating impact of the attacks on the organisation, he said: "They've bent us but they haven't broken us. We'll be back but it will be a very long road. Some of our guys just don't know what's ahead of them in that recovery."
Mr Whelan came to New York a week before the assassination of President Kennedy. In his years as a fireman, more than 150 of his colleagues have been killed in action up to September 11th.
"Since it started 150 years ago, the fire department has had 780 deaths, before this. The number of firefighters who have died stands at 345." The numbers are so high, he says, because the attacks happened just as there was a shift change and people leaving got back on the fire rigs as they pulled out.
He speaks of Danny Suhr, whose parents are Irish. "We buried Danny on Sunday and his family got his body back, so they will have closure. But other families, they can only hope." Danny, although injured, saved seven other firemen.
"One of the 'jumpers' landed on him and he was seriously injured. Seven other firefighters brought him a block down the street to an ambulance and that was when the tower collapsed."
Their icon was the New York fire department's chaplain, Father Mychal Judge (68), whose father was from Co Leitrim. The Franciscan priest had rushed to Ground Zero and died giving the Last Rites to a fatally injured fireman, when he was hit by rubble as the tower collapsed.
"To most of the firefighters he was a walking, living, breathing saint," says Mr Whelan.
Among the dead were the most senior members of the fire department including the fire chief, Mr Peter Ganci, and the first deputy fire commissioner, Mr William Feehan. Such were the losses that the department was forced to immediately promote 168 firefighters to rebuild the management ranks. The Irish names were there too.
The New York City fire department is an essential part of the city's services and its political influence can never be underestimated. "It is a strong Irish labour union," says Mr Whelan, who stresses that the Emerald Society is a fraternal, cultural organisation. Potential city mayors and officials seeking election seek to have its support; the Irish arriving in New York in the earlier years of the last century built the backbone of the city's fire service.
New York's citizens have through the years acknowledged the Irish dimension of the department and in the days since the attacks have hailed all the firefighters as heroes, who gave their lives to protect the community.
Outside almost every fire station in Manhattan are shrines. Rows of candles a foot deep stand in front of permanently refreshed bunches of flowers with framed pictures of the dead and messages up such as "God Be With You - We Love You".
Their heroism is seen even more in the return of firefighters to assist in the recovery operation and find the remains of their colleagues, knowing that as they search through the rubble they could find body parts.
Such is their connection to each other that the losses at one station left 30 children without their fathers. "We will all adopt them and they will be all our children," says one fireman.
Many of the firefighters involved in the rescue are sons searching for their fathers and vice versa. There has been something of a tradition for young men to their fathers into the New York city fire department. The majority of the Irish are second, third and older generation Irish-American.
When Mr Whelan arrived in New York in 1963 there was a big intake of Irish-born. Fewer Irish got involved in the 1970s and 1980s, but the numbers have increased in the past few years.
The impact of the deaths is just beginning to hit a venerable organisation which will have a huge job in rebuilding the experience of generations of firemen.
One of those heroes was Timmy Stackpoole, an Irish-American who was seriously injured in a fire three years ago. "His legs were burnt to the bone and he had to go through skin grafts and physio," says Mr Whelan. He went back to work with his unit just two months ago, and last week he was presented with a plaque for "Irishman of the Year".
He was buried at the weekend.