A most unusual situation happened to a friend: they were travelling Ryanair, checked in fine and in good time, with checked in luggage, queuing for boarding. Suddenly the airline just announced "actually that's it folks, the plane is full, if you didn't make it, tough, come see us for a replacement flight", with people still in the queue.
My friend went over to Ryanair desk, who did put her on another flight, flying to a totally different part of the destination country - a good few hrs travel. They did have the option, at that point, to fly to the original destination, however ultimate arrival time would be much later than flying to a replacement destination+transfer/connecting flight. Initially, my friend hoped to fly Ryanair from that town to her ultimate destination, but as the replacement flight itself was delayed (joint effort by Ryanair and slow luggage handling at the replacement destination) they missed that, as well as any other viable transport links (trains etc). They have some friends in the replacement destination, so stayed the night and travelled onwards safely the next day.
However, it seems like Ryanair would be liable for some compensation. The most glaring fault would appear to be not letting a checked in passenger on board, with no compensation, and leading to a nearly 24h delay of the total journey. Even had they switched to a flight to the original destination, the total delay would be around 12hrs, assuming everything else goes to plan. Further, even though my friend stayed with friends, they are out of pocket for the train ticket they bought the next day.
Do you have any advice on what specific claims can be made, and in particular how to make them against the giant of customer service that is Ryanair?