I would enter only Brighton-Holyhead in the app. That will be your outbound trip. For the ferry you might get a discount (?) but you don’t have to enter it in your app. Might be just that they ask to show your Interrail pass as proof for the discount.

THE ferry is Stena, goed direct from right beside Holyhd trains and also gives pass-discount. DO prepare yourself for by now rather not overly train-friendly transfers on the Irish side.

Or for a little more comfort-and a higher price-you could use the (if it still goes) overnight (that would be Irish F) Liverpool-Dublin. The OUT trip in UK is also valid for that.

@mcadv  Stenaline stopped the discount for Eurail/Interrail :/

@seewulf  It's back :)

This is some very inaccurate info. Firstly both Irish Ferries and Stena Line sail Holyhead - Dublin and both offer some discount to passholders. Both offer 4 sailings each way daily. Stena have 2 large ships, IF have 1 large and a smaller fast ferry that only runs in high season, April-September this year. IF also have a second conventional ferry (Epsilon) but that never takes foot passengers so can be ignored.

There are 2 companies sailing Dublin - Liverpool but they don’t and have never taken foot passengers.

A shuttle bus which  runs to/from the 2 ferry terminals in Dublin port, it runs to Dublin Connolly station. This connects with all sailings except the 23.45 Stena arrival and 02.15 Stena departure.

Of note, while the 20.30 Stena and 20.45 IF departures from Dublin have a bus connection from the city they are poor options for onward travel as there is a long wait (4 hours) for the first train from Holyhead with very poor facilities available in the station or the town. Anyone wanting an overnight connection is best advised to take the 02.15 Stena sailing, paying €10 for a taxi is worth avoiding an uncomfortable nighttime in Holyhead. Walking to/from Dublin port is possible but not recommended, it is a large industrial area with virtually no pedestrian traffic.