From the BBC news (Northern Ireland) website:
Ryanair said in a statement it was due to an "error by the Eirjet pilot who mistakenly believed he was on a visual approach to City of Derry airport".
The 39 passengers were taken by coach from Ballykelly to the airport.
In its statement, Ryanair said it had notified the aviation authorities in the UK and Ireland of the incident.
"We have also asked Eirjet, the operator of the aircraft to carry out a full investigation into this matter, as in over seven years of Ryanair flights into City of Derry airport, and over 20 years of Ryanair-operated flights, such a mistake has never occurred before," it added.