Dublin you can take a taxi or ship shuttle to town then take the Hop On Hop Off bus around Dublin.Easy walking tour also in the centre of the city.
Holyhead I would do ship's tour or if you manage to dock see if there are local taxis
We were unable to go ashore as the waters were too rough so we missed the port.
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012