Belfast couple Robert McKenzie and his wife, Wilma, had just come out of the sea when  a gunman opened fire at a beach resort in Sousse.
Three Irish people and at least 15 British were among 38 people killed.
The couple were staying at a hotel about 300 yards from where the shooting happened.
Mr McKenzie said "pandemonium" broke out.
"Everyone was running. I hadn't heard any gunfire," he said.
"I didn't see anything. I didn't hear anything other than people running and I saw horses running back towards us. Where the incident happened, I understand, is three hotels down from us. It was close enough, and close enough to get everyone off the beach on our particular section of it."
He said he had subsequently been told that people had been shot dead.
"We were very upset but the people of Tunisia, all the hotel staff that we have known over the years were very, very nice and they kept apologising to us, they were in tears, they are such beautiful people."
Mr McKenzie said they were very upset by what had happened.
"If they had come to our section of the beach, we'd have been wiped out because we always lay right at the front of the beach and we're never off the beach and wouldn't have had a chance, quite honestly," he said.
"By the grace of God, it didn't happen to us, God was looking after us."
A couple from County Westmeath and a woman from Robinstown, County Meath, are among the dead.
Irish broadcaster RTÃ‰ said the couple had been named locally as Laurence (Larry) and Martina Hayes.
They were in their 50s and came from Athlone. They had one daughter.
Irish woman Lorna Carty, a mother-of-two from from Robinstown in County Meath was also killed.
The Irish government is warning people travelling to Tunisia to "exercise extreme caution".
A Northern Ireland man said he and his wife would have been "wiped out"  in the terror attack in Tunisia had they been on a different part of the beach.