X1 isn't really an express, it is the renamed 1/200 service operated jointly with Bus Eireann, as well as Dublin Airport, Dublin City and Belfast, it stops at Newry, Banbridge and Sprucefield. Operated mostly using 1 year old VDL double deckers on the Bus Eireann side and even newer Scania I6's on the Translink side, it operates every hour, 24 hours a day during the summer but in the winter some of the early services do not run. All offer Wifi on board.
X2 is a true express, from Dublin it runs from Dublin Airport to Belfast City only. but curiously in the other direction it runs all the way from Belfast City to Dublin City. There are no other stops apart from these. It is usually run by Scania I6's which were new to Translink in the last few months. It isn't as frequent as the X1 and operates (mostly) every two hours and most of the services are summer only. All offer Wifi on board.
http://www.buseireann.ie/pdf/1373031833-1X.pdf
If you are looking to go from Dublin City to Belfast, you may want to consider Aircoach, who operate 22 times daily in each direction on the route (basically hourly apart from late evening where it goes to every two hours) All services are express in both directions, serving Dublin City, Dublin Airport and Belfast City only. Can see
interior of coach here and can be as cheap as £8 if booked online. All offer Wifi on board.
http://www.aircoach.ie/table.routes.belfast.php
Also consider the train, since if you book seven days in advance you can get prices from €14.99 from the Republic of Ireland end, or cheaper still if you buy them from Translink.
http://www.translink.co.uk/Services...-Web-Fares-Enterprise-Promotional-Web-Fares-/