I've done the Euston Dublin and LGW/LHR DUB journeys several times as I had a family member who hated to fly. There really is nothing of interest to see on the journey and I would always fly for choice. You may find fares are substantially less with Ryanair from Gatwick-a very basic flight but for less than an hour's flight it would be worth considering www.ryanair.co.uk .