It is in an airline's interest to deny boarding to a passenger if possible, albeit in a very cynical way. In some cases, this is because they have overbooked. (They deny that they overbook, but an unsubstantiated claim from someone purporting to be ex-staff of Ryanair claims that Ryanair DO Overbook Their Aircraft).
But the more likely reason an airline will deny you boarding is that they know you need to get to where you're going. They know you will pay for a new flight. Charging one passenger twice for the same service is good business sense, wouldn't you say? In fact, most passengers already pay twice for a service with Ryanair, as they pay multiple credit-card fees, despite the fact the purchase is a single transaction.
Ryanair is more likely to deny boarding than other airlines because it seems, they are less afraid of the bad press than other airlines. They are also lucky that they are based in a part of Ireland that has a very bad postal service - many complaints never even reach their offices!
Reasons for Denied Boardings
Note that not all of these reasons are unique to Ryanair. Not included are the obvious reasons you might be denied boardings, such as drunkenness, intoxication or verbal or physical abuse towards Ryanair staff.
- Pregnant passengers who do not have a doctor's note in English
- No Stamp on Boarding Pass (non-EU citizens)
If you are from outside the EU, you must have your boarding pass stamped by a member of staff at the information desk. If you proceed directly to the gate without this, you will be denied boarding.
- Oversized Luggage
Ryanair has a strict baggage policy, with hand luggage dimensions the joint lowest in Europe (along with Wizzair). At many airports, you are not allowed to carry any "duty-free", unless it fits in your hand luggage (easyJet also does this at some airports). Normally you will be able to pay a fine to carry these items on, but if their credit card machine is broken and you have no cash, then you will be denied boarding.
- Slight Damage to Passport
This happened to me. There was a slight tear on the passport page. Their airport authorities told me the passport was perfectly legal, but the airline can deny boarding at its own discretion.
- Traveling with Local Form of ID not Recognised by the Airline
Ryanair doesn't believe in following local laws, only Irish law. Therefore if you have a form of ID legal in your country, if Ireland doesn't recognize it, you will be denied boarding. A Spanish family successfully sued Ryanair when they were denied boarding on an internal flight in Spain, for trying to travel with their children on a Spanish Family Card.
- Passengers Traveling with a Driving License on Internal UK Flights
For internal flights in most of the EU and for flights between Schengen countries, a national ID card is a sufficient identification. But a problem arises for UK passengers on internal UK flights. The UK does not have ID cards. Many airlines (including EasyJet and Flybe) accept photographic driving licenses.
- Deaf or Blind Passengers, Traveling Alone and Who Haven't Advised Ryanair
This is actually an improvement on Ryanair policy: before, they only allowed four blind passengers in total, regardless of whether they were accompanied or not.
- The Fifth Sole Traveling Passenger Requiring Assistance Even if you have advised the airline, if there are four or more passengers who require assistance, only the first four will be allowed to board.
- Traveling with a Guide Dog on Certain Routes
- With a Leg in a Cast
Passengers with their leg in a cast must purchase three seats or they will be denied boarding.