"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.
From the web
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? firstname.lastname@example.org