I've seen big cruise ships in the harbors of Iceland when I've been there. Now I'd like take a cruise somewhere from/to Iceland and wonder: what cruise ships stop in Iceland?
Where are some resources to research this online?
You asked: How, not Which, so I explain how I would search for such cruises:
I would use CruiseCritics: for example the advanced search in: https://www.cruisecritic.com/cruiseto/
Then you should put as port of call a port in Iceland. I would choose Reykjavik, as starting point. Wikipedia could help to find major ports, but take into consideration that cruise lines will often use different cities (with possibly just in parenthesis the real port name). This make very difficult to search in the very long drop down menus.
Then you submit your search (maybe specifying period and style). Pass quickly the cruises, and check if one do various stops in Iceland. This will give you hints for additional port-of-call searches. But in this case, I assume that nearly all cruises passing for Iceland, will reach also Reykjavik.
In such manner, you have the list of cruises that stop in Iceland. Note: reading the result, you will find some duplicates: this because there are not "loop" cruises (every week or two weeks repeating the same route), and so the cruise companies will offer you various combined cruises (to Iceland, from Iceland, combined, half or entire world cruises).
In my experience, cruise critics (but the forum, ask there if you need very specific questions) tend to feature cruise ships, and not ferries which offer also cruise style cabins (but not all cruise facilities). If your cruise style is ok for such type of travel, you can navigate in many more small ports, and meet Icelanders on the ferry.