The Multicrew Pilot Licence is exactly that - it's a licence which allows you to act as part of a multicrew environment. The traditional CPL/IR is a combination of modules which can result in you flying in a multicrew environment.
The MPL course is one that is specifically tailored to a particular airline, for this reason it must be undertaken at one FTO with an airlines involvement ie. The Flybe/OAA MPL currently running.
Unlike the CPL/IR, the MPL is tailored from the start for the airline that the cadet will work for so cadets on the OAA course will be using Flybe SOPs for a large part of their training.
The MPL also involves less single pilot flying - the licence is designed for working in a flight deck so a large proportion of the training is done like that (in a simulator).
Apart from the groundschool which is exactly the same for both licences, the two routes to the flight deck are very different.
While the MPL does provide a more secure route to the RHS (providing the sponser airline is reasonably stable - Flybe vs. Sterling) the CPL/IR is still the route taken by the majority of students.
There have been numerous debates about which licence is better and many people are set in their ways regarding the CPL/IR however I personally feel that the MPL is catching on and may one day be an equally popular route to the flight deck providing that airlines remain keen to support it.