Organisers of the marathon said the man was treated at the scene by St John’s Ambulance, who then transferred him to nearby St Vincent's Hospital, where he is now in a serious condition.
It is understood he was among the thousands of runners crossing the line after four hours running.
"We are very concerned about his wellbeing," organisers said.
"He was a competitor and he collapsed at the finish line. Members of St John's Ambulance treated him and he was taken to hospital almost immediately."
"We are staying very much in touch with the hospital to keep up-to-date with how he is doing," they said.
The man's difficulties put a dampener on what had been - until then - a feelgood event for Ireland, as the country claimed the winner in both the men's and women's events for the first time in 20 years.
Primary school teacher Sean Hehir (28), from Kilkishen, Co Clare, landed the race in 2:18:19 on a cold St Jude's day, holding off the advances of 6ft 2in Joe Sweeney who finished in his wake.
Sean's mother Cushla Murphy-Hehir told how she had not been able to sleep the night before the race due to her nerves.
Her son was taken to the elite massage tent to try and recover from the gruelling 26.2 miles trek across the city.
"He has trained very very hard," said the proud mother, who was also a runner alongside John Treacy in the '80s.
"He has trained twice a day - 6.30am and 6.30pm. He did altitude for three weeks in the summer. This is his second ever marathon he was second of the Irish last year," said Mrs Murphy-Hehir.
The Dublin-based primary schoolteacher won after a fantastic two-man tussle with Dundrum South Dublin's Joe Sweeney.
The two slogged it out in a fascinating duel for most of the race.They went through half-way together in 68 minutes and 25 seconds, which left them headed for a sub-2:18 run but the second half took its toll on both of them.
They've been side-by-side for the first half of the race but, at mile 14 debutant Sweeney looked the stronger when he made a decisive move to open up a quick nine second-gap by throwing in a quick 5.08 mile.
But Hehir caught up with him and made his own big move in the 20th mile through Clonskeagh, pulling clear going up Milltown Hill and never looking back.
It was still a great marathon debut for Sweeney, who finished less than a minute adrift in 2:19.26 and long-time Irish resident and Moldovan international Sergio Ciobanu (Clonliffe) was third in 2:22:02.
Sweeney's DSD clubmate Maria McCambridge was the big women's pre-race favourite and led through half-way in 79 minutes and 25 seconds but Cork's Claire McCarthy-Gibbons (Leevale) overtook her in the 18th mile and the two women had a fantastic tussle over the final six miles.
Defending Irish champion McCambridge somehow managed to battle back and retained her title in 2:38.51, with McCarthy second in 2:39.27 and Tullamore Olympian Pauline Curley was third in 2:42.58.
Earlier,hopes were high that an Irish runner might be first person past the finishing post at Merrion Square North for the first time in 20-years.
Earlier this year, in the absence of a headline sponsor, Dublin organisers were forced to cut the elite race fund, which meant the usual invitations were not issued to some of the East African runners who have dominated in recent years.
Race organiser Jim Aughney revealed people had still travelled from far and wide to take part in the race, with runners from 47 countries taking part.
Mr Aughney told how the popular race had attracted around 2,000 from the UK, around 500 from the US and a group of 12 over for the first time from the Philippines.
In addition, he told how hundreds of thousands of euro would be raised for their two chosen charities Temple Street and Focus Ireland, with many other runners picking their own personal worthy causes.
"We were sweating for the last couple of days," he said, adding it had been a "serious" day as they tried to erect the marques and signs.
More than 10,000 Irish people took part in the 26.2 mile race and a further 4,000 representing around 47 different nationalities travelled to the country.
Welsh National Champion Alun Evans was the only elite international male runner to participate.
Members of An Garda Siochana and the Police Service of Northern Ireland joined forces for the fifth year running and took part under the Garda Athletics Club vest.
The officers, who raised funds for different causes close to them, were cheered on by the Garda Band and the Assistant Commissioner for the Dublin Metropolitan Region.