"We all laughed but then 10 minutes later they actually did it. They seemed so drunk, they brought a lot of attention to themselves."
Williams said the couple wasted no time getting down to business.
"You could see the girl taking off her pants and he pulled his trousers off. She started riding him," he said.
"I had to get my phone out. I have never seen anything like that."
Williams alleged cabin crew did nothing to stop the couple or even reprimand them, and disgusted passengers who asked to be moved to another seat weren't able to do so.
"A woman was sat next to them and she stood up and asked if she could be moved. About 20 minutes after it happened, someone complained about it. But the crew did nothing," he said.
A spokesman for Ryanair told the Mail Online the budget carrier was "looking into this matter".
"We will not tolerate unruly, disruptive or inappropriate behaviour at any time and any passengers who appear to behave in an unacceptable manner may be liable for further sanctions," the spokesman said.
The couple probably should have booked their flight instead with Flamingo Air - an airline that said to be the only one that actively encourages people to have sex mid-flight.