Hi! I've been curious about the Greek islands too.
As far as I know, the global pass does not cover the ferries to most islands, but there is a separate pass that does cover various of those ferries: https://www.interrail.eu/en/interrail-passes/one-country-pass/greece-passes/greek-islands The global pass does give 20% (Minoan Lines) or 30% (Blue Star Ferries & Hellenic Seaways and Blue Star Ferries in cooperation with ANEK LINES to Crete) discount on ferries to the islands, of which a few would be free with the Greek Island pass.
The global pass does cover the ferry between Italy and Greece on Superfast ferries/Anek lines, but it is not entirely free: Some port fares and high season supplements (if applicable) do have to be paid, and - if chosing a cabin - that has to be paid too. It can be booked online through superfast.com, and - from the few ferries I've checked I've found them to be surprisingly expensive in the end. Cheapest option with a bed is €75 per person (in a 4-bed cabin with a 1st class global pass).
The Interrail information page on this ferry lists the following routes from Italy:
- Bari (Italy) – Corfu (Greece) (sailings during summer period only)
- Bari (Italy) - Igoumenitsa (Greece)
- Bari (Italy) - Patras (Greece)
- Ancona (Italy) - Corfu (Greece) (sailings during summer period only)
- Ancona (Italy) - Igoumenitsa (Greece)
- Ancona (Italy) - Patras (Greece)
- Venice (Italy) – Corfu (Greece) (sailings during summer period only)
- Venice (Italy) - Igoumenitsa (Greece)
- Venice (Italy) – Patras (Greece)
(I haven't checked if the list is up to date)
Corfu is obviously an island (but just one), and Igoumenitsa does not seem to provide further connections to other islands, leaving only Patras as interesting for onward travel. From Patras a bus (operated by the Greek Railways TrainOSE, Interrail should be valid but not sure) + train combination allows reaching Athens, from where ferries leave to many of the islands.
Unfortunately, there currently are no international trains from Greece either, so reaching it over land would also require a part by bus. I don't really know much about that.