The airline will probably require proof of identity when you check in. A passport is a good way to provide this.
Here are Ryanair's requirements:
All passengers must present valid photo identification at check-in for all flights. The only acceptable forms of photo-ID on Ryanair flights are:
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, Finland, France, Germany, Greece, Cyprus, Czech Republic, Estonia, Hungary, Italy, Liechtenstein, Lithuania, Luxembourg, Netherlands, Malta, Poland, Portugal, Slovakia, Slovenia, Spain, Switzerland.
A valid driving licence with photo (in the case of UK domestic flights and UK-Republic of Ireland-UK routes only)
N.B. Children under 16 years (including infants) travelling with an adult are required to have photo-ID when travelling on flights between European Economic Area (EEA) countries. Acceptable photo-ID for children under 16 is either a valid passport or a valid National Identity Card issued by an EEA country.
Photo-ID for children under 16 years is not required in the following circumstances:
Children travelling on UK domestic flights and UK-Republic of Ireland-UK routes.
Children included on the passport of the parent with whom they are travelling.
German children under 10 in possession of a valid German Government Kinderausweis travel document.
In accordance with Article 28(1) of the 1951 UN Convention, a valid UN Refugee Convention Travel Document issued by a Government in place of a valid passport is acceptable.
Expired forms of photo-ID will not be accepted for any flight. Failure to present valid photo-ID matching the names on your reservation will result in your being refused check-in without refund.