British Ms Lessing, 88, who won the Nobel Prize for literature earlier this month, said the attacks were not as “extraordinary” as some Americans think.
She told Spanish newspaper El Pais: “September 11 was terrible, but if one goes back over the history of the IRA, what happened to the Americans wasn’t that terrible.
“Some Americans will think I’m crazy. Many people died, two prominent buildings fell, but it was neither as terrible nor as extraordinary as they think.
“Do you know what people forget? That the IRA attacked with bombs against the [British] government.”
Nearly 3,000 people were killed in the September 11 attacks of 2001.
More than 3,700 died and tens of thousands of people were injured in more than 30 years of violence in Northern Ireland.
Ms Lessing, author of The Golden Notebook, also poured scorn on former British prime minister Tony Blair and US President George W Bush.
“I always hated Tony Blair, from the beginning,” she said. “I think he has been a disaster for Britain and we have suffered him for many years.
“As for Bush, he’s a world calamity. You have to remember he is a member of a social class which has profited from wars.”