Hi Andrew. 

Anek Lines are the better service. You can go from Ancona to Patras or Bari to Patras. It takes 24 hours approx. Best to book a berth if you can. You get a reduction with your pass. 

When you book you see a section on the booking page entitled “Offer”. Select your pass type Interrail Greek Islands might be an option or Eurail. 
Here’s a link: https://www.anek.gr/booking/#step2

Then click “Price”  and you will see a reduced price fare. 
https://www.anek.gr/booking/#step2

it is well worthy getting a berth if you can afford it. It is a lovely journey.m, have done it twice. Remember Anek are the better of the 2 companies on that route.

No trains in Patras but buses to the nearest train station. Look at Seat61 website for info.

best wishes, Joanne