I watched the episode this evening and I enjoyed it. If you're looking for detail about the railways, that is not this shows intention; he is following Bradshaws book of the same title, which isn't a book on railways, but rather a guide on the locations that can be reached by rail and a description of the journeys involved which is what Portillo is doing.
As for 'Great British Railway Journeys' being in Ireland, the book was written in the 1860's as far as I know when Ireland was still very much part of the empire and to be fair these Irish shows are titled 'Great British Railway Journeys visits Ireland', to keep everyone happy