Can I travel to Milan from Brussels without a passport, only 5 years residence ID?

For Ryanair's documentation requirements, you should check their terms and conditions.

• A valid passport (Note: all non EU passport holders, travelling into a Schengen member country are obliged to ensure that their passport is valid for at least 3 months from the date of their departure from the Schengen member country. This requirement does not apply to holders of a Schengen issued residence permit or long term visas).
• A valid National Identity Card issued by the government of a European Economic Area (EEA) country. (Only the following EEA countries currently issue National Identity Cards acceptable for carriage on Ryanair flights: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Estonia, Finland, France*, Germany, Gibraltar, Greece, Hungary, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Netherlands, Malta, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland. *The validity of French National ID cards (issued for adults) has been extended from 10 years to 15 years ONLY for cards issued between 2nd January 2004 and 31st December 2013 (except flights to/from Malta) .
• Irish Passport Card - the Irish passport card can be used by Irish citizens for travel within the European Union and the European Economic Area.
• For travel to Morocco - only a valid passport is accepted. The passport must be valid for the period of intended stay.
• For travel to Israel - passports must be valid for the period of intended stay and passengers must be in possession of a return and or onward ticket.
• For travel to Jordan only a valid passport is accepted. The passport must be valid for six months beyond the period of attended stay
• valid German Government issued ‘Kinderausweis’ travel document
• A valid Greek National Police identity card
• A valid Italian ‘Certificato Di Nascita’ with photo (for use by children under 16 years) which has been endorsed as ‘VALIDO PER L'ESPATRIO’ for travel on international flights, . A
• A valid UN Refugee Convention Travel Document – (issued in accordance with Article 28(1) of the 1951 UN Convention, by a Government in place of a valid passport.)
• A valid Convention Travel Document – (issued in accordance with Article 27 of the 1954 UN Convention for Stateless Persons, by a contracting state in place of a valid passport)
• A valid Collective Passport issued by an EU/EEA country
• A passport for travel outside the EEA (European Economic Area) must be valid for period of intended stay.
  If a visa is applicable any children/infants must be travelling with the adult named on a visa vignette

From that list you can see that Ryanair only accepts passports and EEA national identity cards. A residence card is not accepted. As a non-EEA citizen, you must have a valid passport to fly with Ryanair.