First time is a charm

Unfortunately Spain and especially Portugal are badly linked to the rest of the continent. Lisbon - Rome would be at least 3 full travel days.

For a 20 day trip I'd either drop Portugal or fly from Lisbon to Rome/Naples. Then it becomes really manageable and a cool experience. :)

For example you could take the fantastic Bernina Railway between Italy and Switzerland: https://www.seat61.com/trains-and-routes/bernina-express.htm

Then the Eurostar across the Channel is incredibly convenient (although popular so book in advance).

There are multiple ferry routes between Great Britain and Ireland -> you get 20-30% discount with the pass.

Have a look at these links and don't hesitate to come back with specific questions:

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm