If you are comfortable driving, it will open up a lot more of the country to you.
In
Ringaskiggy, check out Great Island car rentals:
https://www.greatislandcarrentals.com/. You can pick up and drop off your car right at the cruise terminal. There are direct buses from the port to Cork, while the train station is about a 20-minute walk away.
While most British Isles cruises don't allow you to visit the best Irish attractions - those in/around Dublin, along the Ring of Kerry, or at Skellig Michael - Cork is still a great city to tour. You can go all the way to Blarney Castle past Cork. While Cobh is charming, I wouldn't spare a whole day in town when you are docked in Ringaskiddy.
From
Greenock, you can catch the train to Glasgow or Edinburgh. Edinburgh has much more history to it, but the train takes about two hours in each direction. Once in Edinburgh, you can explore the Royal Mile and old town very efficiently.
Belfast has the Titanic Museum, Ulster Museum, gaol, etc. With a car, you can the visit the Giant's Causeway in the morning and then stop in the city in the afternoon on the way back.
Liverpool has a lot of Beatles memorabilia as well as a football/soccer theme, which you can pursue if you have interest. The city has its own museums and cathedrals, or you can drive to Manchester or the Lake District/Peak District national parks.
The port of
Portland doesn't allow you to walk around on your own, and there isn't much to do on the peninsular island. Best to take a tour to a nearby attraction - Stonehenge, Jurassic Coast, etc. If you can find a rental or a private driver, check out the Lulworth Cove Cliffs on the Jurassic Coast.