It was only after landing and being greeted with "bonjour" at passport control the couple said they realised they were in Beauvais, France.
The airport, just outside of Paris, is around 1200 km from their intended destination.
Ryanair did not provide a comment to Manchester Evening News but stated "it is each passenger's responsibility to ensure they board the correct aircraft.
"There are several touchpoints throughout the passenger journey which inform passengers of the aircraft's destination."
Examples included the flight number and destination displayed at the gate, during the welcome PA on board.
Forster said no announcements were made in English. "The shock of landing somewhere near Paris when you're supposed to be going to Copenhagen was ridiculous," he told the news outlet.
"I would like to understand how on earth this happened."
Forster claimed he contacted Ryanair the morning after landing in France on December 18 and requested reimbursement for his flights and hotel in Copenhagen and was given tickets to fly home.
He was directed to an online complaint form, which he filled in on December 21. After staying in France for the weekend, the couple booked return flights home to the UK with Easyjet.
"We had a lovely weekend and on the way back we kept getting messages from Ryanair about the return Copenhagen flight being delayed," Forster said.
Despite several attempts to get in touch with Ryanair, Forster said he had been unable to get in contact with someone to discuss reimbursement for flights and €1,100 (NZ$1900) incurred costs.