Hi Maestro1981,
Probably you we're not allowed to board due to a missing visa stamp (on the boardng pass), which usually can be obtained before going through airport security. Only low-cost airlines (Ryanair, Wizz Air, etc) have such check for flights within the Schengen area.
IMPORTANT INFORMATION FOR ALL NON EU/EEA PASSENGERS
Irrespective of a passenger's visa requirements, all non EU/EEA citizens must have their travel documents checked and stamped at the Ryanair Visa/Document Check Desk before going through airport security.
Non EU/EEA passengers must do this in order to ensure compliance with immigration authorities. We reserve the right to refuse travel, if there is a failure to comply with this requirement.
If you have been denied boarding and require immediate clarification please call +353 1 2480860 to speak to one of our team (subject to opening hours).
Source:
https://www.ryanair.com/gb/en/useful...orts-and-visas
Wizz Air, another low-cost airline in Europe has similar regulations