Tsarnaev, 19, is believed to have been hiding in the boat in Watertown for hours before he was noticed.
Donegal man Brendan Toye was in lockdown next door with his wife and young son.
Brendan from Carrigart said: “We were sitting in our house thinking we were safe watching all this on television. We didn’t know he was just a few feet away hiding in the boat.
“He was armed and he could easily have jumped over the fence. I don’t like to think about it but he could have killed us all.”
The first Brendan knew that the bomber was next door on Franklin Street was when armed police officers knocked on his door just after 6pm on Friday.
The officer grabbed the couple’s six-month-old son Garbhan and told the couple to follow them.
“He said he presumed we knew they were looking for the bomber and they thought they had him cornered next door.
“I was frightened but my wife Lori was terrified. It was all very surreal.
“We grabbed our coats and we were out the door in seconds,” said Brendan.
They ran down the street and took shelter in a house down the road.
Minutes later Brendan said he could hear rapid gunfire followed by a series of up to four controlled explosions.
Seconds later there was a huge round of applause as locals realised the bomber had been caught.
“I think they were using by house to shoot at the suspect in the boat.
“It’s the perfect spot as it looks right across to the boat where he was hiding — you can almost touch it,” said carpenter Brendan who moved to America from Donegal 12 years ago.
“It’s just one of those things but I am very conscious that three people were killed and a lot more badly injured by this atrocity.
“Watertown is a quiet area and we only moved into this house in January. I have been doing the house up and we’re very happy here.
“This could have happened anywhere but it was just unfortunate that it was next door to us.”