Interested in comments re best options for sightseeing when in ports Irish ports of Cork, Dublin, Belfast, and also Holyhead Wales. Is it better to plan excursions separately or go with our cruiseline (Celebrity) offerings?

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.