my two cents: what do you want?
portmarnock = world class, very fair (only one truly blind shot), flattish dunes, surrounded by water on 3 sides... one of the best par 3's (the 15th) in world golf...
the island = slightly rougher, high dunes, the odd quirky shot, slightly less fair, some big wow factors, surrounded by water on 3 sides, not as good a course as portmarnock
haven't played royal dublin since hawtree's changes but by all accounts it is a different course, 16 new green complexes, some new holes, different routing... however, the person who said it was an industrial setting is being a bit unfair... sure, there's some chimneys and a plant over the water but RD is situated on a nature reserve island surrounded by water on ALL sides... it's certainly a worthy third choice to the other two and is much more central and easy to get to...
architecturally speaking though, portmarnock is the must play...