No, it's not only for European citizens...They probably were just thinking that the majority of the people flying it would be Europeans.
I found this on their site:
All passengers must present valid photo identification at check-in for all flights.The only acceptable forms of photo-ID on Ryanair flights are:
WHAT PHOTO-ID DO I NEED?
* A valid passport
* A valid Government issued National Identity Card issued by a European Economic Area (EEA) country. Only the following countries from the EEA issue National Identity Cards which are acceptable for carriage: Austria, Belgium, Bulgaria, Finland, France, Germany, Greece, Cyprus, Czech Republic, Estonia, Hungary, Italy, Liechtenstein, Lithuania, Luxembourg, Netherlands, Malta, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden (for travel within the Schengen area only) and Switzerland.......
So, that tells me it's more like an National ID card for EU?EEA citizens or any passport.
My husband and I are Americans and we have flown with them.
The think you do need to remember is MANY of their flights DO NOT go to major airports. For example, if you go to Stockholm, it's not the major international airport you would usually fly into but another one about 100km or so outside of the city. Just be aware of that.
Otherwise... it's a pretty good, cheap airline.