I would say the big irony about the visit that Charlie makes to his mother lies in the way that, to a certain extent, their conditions and roles have been reversed. Now, instead of Charlie's mother having to look after her son, it is Charlie who is intelligent and having to look after his mother. She shows that she is losing track of reality, and at one stage, after having recognised Charlie, suddenly thinks he is in her house to ask about the electric bill. However, the biggest irony comes when Charlie asks her about her children, and she tells Charlie (who she does not recognise to be her son at this stage) about her son:
"I had a boy. So brilliant that all the other mothers were jealous of him. And they put the evil eye on him. They called it the IQ but it was the evil IQ. He would have been a great man, if not for that. He was really very bright--exceptional, they said. He could have been a genius..."
The irony of this conversation is of course that, in spite of his mother's dreams of Charlie and the way that she obviously ignores reality, Charlie's mother is right: her son has become a genius and has shown himself to be truly exceptional, thanks to the operation that has become both a blessing and a curse to Charlie.