Treat the chimney breast as a separate item - Paper up to the internal corners on each side left and right and allow a small overlap onto the side walls of the breast - an inch perhaps.
Decide where you want the paper on the front of the breast i.e. join up the centre or somewhere else and paper the front face - go round the corner and finish at the internal corner - you will not get a match but it usually does not matter - nobody will look at these internal corners but the front face will always be in plain view.
Always hang paper as vertically as you can and let any irregularities and out of square issues disappear in hidden corners. You will only notice them during the paperhanging - other times they're unseen.
Good luck...
Peccavi
December 2009