Sean Mulleady (49) and Pat Gaffney, who is in his late sixties, died in the freak accident.
Two investigations were under way today into the tragedy, which occurred at 1.30pm yesterday afternoon.
Speaking to the Herald today, Ms Fallon said the two deceased "simply hadn't a chance. It felt like the world turned upside down in a second."
"I looked across the shop and I saw Pat [Gaffney], who I grew up with, in a really bad way," she said.
"I screamed at Bernard to go and help him. He rushed over and lifted five or six large bricks off him.
"But I knew he had been injured very badly and I rushed outside to tell his wife what had happened.
"It's the most upsetting and heartbreaking thing I've ever had to do. What happened yesterday was a nightmare."
Ms Fallon, from Lanesboro, Co Roscommon, said today that the wall collapse "happened in seconds" and sparked "hysteria".
"I looked around after hearing a massive gust of wind hit the shop and I saw the wall coming down and I just screamed at Bernard to run.
"And then it happened. The wall fell down in front of our eyes. It's not something you can describe.
"Some of the bricks fell just yards from us. We are lucky to be alive today.
"We could see blood, we could see people injured. We, along with some of the staff, did all we could to help.
"There were people just lying on the floor in pain. Blankets were being pulled down, we were all trying to lift bricks off those who were injured. But the two lads hadn't a chance. It felt like the world turned upside down in a second."
Both Mr Mulleady and Mr Gaffney were rushed to the Midlands Hospital in Mullingar but died from their injuries.
Separate probes by the gardai and the Health and Safety Authority (HSA) were continuing today.
Four other people are still being treated at the Midlands Regional Hospital, one of whom sustained serious injuries.
Mr Mulleady was a respected businessman from the area and was known for his passion for horses.
His cousin and local councillor, Martin Mulleady, said that the family was still coming to terms with the loss: "We are all very cut up about it. He was just picking something up.
"We can't believe it, the wall fell on him and he was badly injured. This is a massive shock to us all."
His wife Teresa and three children, Mark, David and Claire, along with his mother Sadie, were being comforted by friends and family members today.
Mr Gaffney survived for several hours before succumbing to his injuries. His wife Patsy and children John and Denise were also being comforted by family.