Cruising from Barcelona to Casablanca, Las Palmas, Santa Cruz de Tenerife, Funchal, Malaga, Alicante and back to Barcelona followed by land visit to Paris and Dublin. What are good shore excursions to book during the cruise and what are must see sights in Barcelona, Paris and Dublin if staying only 3 nights in each locale after the cruise?

Shore trips, if you can go walking in the hills along the waterways in Funchal, visit the ancient city of Malaga and the main art museum and do some Gaudi in Barcelona you will have made good use of your time.

Dublin.... Book of Kells and the National art gallery are my top two then there is loads of stuff drink/myths of Ireland/churches etc etc

Paris... too much to say

For shore excursions — it can be an advantage to use a company like Shore Trips. They have an online catalogue of shore excursions at hundreds of ports.

Advantages:
(1) less expensive than the cruise line's own trips
(2) group excursions are a much more intimate group than the 40 passenger busses the cruise lines fill up

Take a look at their website for more info: https://www.shoretrips.com/?linkid=ST1437828