Where exactly are you going? There is no roaming charges across Europe across carriers, so the big question will be network coverage.
In Ireland, I would just go with Tesco mobile. In any place, check that the coverage is adequate, and then go for price. Of course, there are plenty of options sold right at the airport, if you want to get operational immediately.
Also, assuming you won't be streaming, then of course you also don't need unlimited data. You can also limit your data usage by loading offline maps of the areas you are traveling to--which also helps guard against coverage problems being a real issue.
Another way to go really cheap is to get an ip phone number, and then just rely on wifi connections for phone usage. That won't help if you are genuinely in transit, but if it's mainly to be reachable by others back home, or confirming the next step of travel, everywhere you stay will have wifi.
Other things you might want your mobile for: taxi (Free Now) / Uber requests, Deliveroo/ Just Eat deliveries when you are exhausted from sightseeing, Grocery delivery if you are cooking in but also maximizing time, hotel/airbnb communications, airline advance check-in...
Some of these apps will be looking for your mobile ID (I.e. Google ID) to be domiciled locally to show up in your app store. Just create a throwaway Gmail account, homed in your visiting country, and all the local services will show up on the same device.
In my own situation, we are former residents and here for the summer. We've had Google Fi since I gave up my work phone, and we love it. And all our US suppliers / banks / etc. don't even know we're gone. DW had Tesco Mobile when I had a work phone. (For those considering this: Google Fi is a US provider, with extensive international coverage. You need to get a SIM and activate service in the US)