Ryanair has opened for the first time hubs outside of Europe: in Marrakech and Fez.

Thus you may reach Marrakech from the following destinations:

UK: London Luton, London Stansted
Spain: Madrid, Seville, Valencia, Girona (Barcelona)
France: Paris-Beavais, Paris-Vatry (Disneyland), Tours, Dole, Bergerac, Marseille
Italy: Milan (Bergamo), Pisa, Rome, Cuneo
Germany: Düsseldorf (Weeze), Frankfurt-Hahn, New starting April 2013: Karlsruhe Baden-Baden, Memmingen
Belgium: Brussels (Charleroi)
Netherlands: Eindhoven

Check on other flight connections: Travel to Marrakech

If you should wish to make a round-trip, you might consider flying to or from Fes:

UK: London Stansted
Spain: Girona (Barcelona), Barcelona (El Prat)
France: Paris-Beavais, Marseille, St. Etienne (Lyon), Nimes, Lille, Nantes
Italy: Milan (Bergamo), Pisa, Rome
Germany: Düsseldorf (Weeze), Frankfurt-Hahn
Belgium: Brussels (Charleroi)
Netherlands: Eindhoven