Michael Roberts, 61, had finished the race and was in the medical tent close to the explosion when the bombs started to go off.
"There was two bangs and I said 'I'm from Belfast – I know that sound – it's a bomb'," he told UTV.
"Then within a few minutes there was pandemonium in the medical tent.
"It was all over in a flash. It reminded me of Belfast. I recognised the sound.
"I am fine now but these things take their toll."
The two explosions, which went off with the marathon clock at 4:09:44, have left three people dead and at least 140 injured.
One of the dead was an eight-year-old boy, US media said.
At least 17 people are critically wounded, officials say, and the injuries include several amputations.
The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry".
A third explosion at the nearby JFK Library was also confirmed by Boston Police.
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