Well, Belfast to (London)Derry can be done by train as well:
http://www.translink.co.uk/Services/...ce-3-Outbound/
(the stretch from Coleraine is supposed to be rather impressive...)
But if you want to get back to Dublin, you'll have to go via Belfast or get the bus:
http://journeyplanner.translink.co.u...0000bf0_TP.pdf