My son and I did 6 countries in 3 weeks by rail, but they were a bit closer (Czech Rep, Austria, Switzerland, Germany, France, Lux). We knew we weren't really experiencing any one country, but rather had some specific things to see and getting a glimpse of a number of different countries was fun.
I'd make a list of things you'd really like to do and see, and then figure out the logistics of getting from place to place and see how it looks schedule-wise. I think we found the German rail site gave the best info on schedules, or maybe it was Austria. For our trips, the trains were incredibly reliable. Not sure how Italy and Croatia would be. But you really need to look at schedules and see whether it's really possible to spend most of the day in a certain city and still catch a train to the next city that evening or the next morning and also have plenty of time for that city.
If you're thinking more to experience those countries rather than jump around, I think you'll be better off to pick a couple areas and spend more time there rather than try to cover all that ground. I think you'd need to be more specific on what you are looking for: cities, wineries, history, views, etc., before you get any real advice. It just depends on how you want to travel. We did it by moderate sized backpacks, and other than the first 3 days in Prague, never spent more than 2 days in one place. That's not for everyone.
Ireland is your real outlier. If you really want to do that, consider starting or finishing there, and flying to/from the continent. Or save it for another trip. Unless you want a hit and run stop in Dublin, you'll want a lot of time here. Otherwise, you can see the countries you want, just realize that you'll be moving around a lot rather than relaxing and soaking in more of the culture.
We initially thought we'd just get a Eurail pass and have a rough itinerary and wing it, but as it got closer we reconsidered. It made more sense to plan it out and take a logical path with shorter trips than to zig-zag around and spend too much time on trains. Also, we booked in advance so we didn't have to get off the train and go looking for a place to stay and probably not have time to really consult reviews before deciding. Or what if we happened on a place with some convention or festival going on that we didn't care about but took up all the lodging? For some places I made sure we could cancel with little or no penalty if our plans changed, especially outdoor stuff due to weather.
You might find a guide book that seems to resonate with you and use it. For us it was Rick Steves. He's a bit goofy on his TV shoes but he goes for the lower cost but safe lodging with no frills, and a more authentic local experience rather than a chain. Lonely Planet is another one you might consider. The more classic guidebooks like Fodor's or Frommer's just didn't seem to be for me. I also use TripAdvisor as a guide. Far from perfect but usually works for me.
