Unfortunately, Irish railways is one of the operators who don't send their timetables to the European Timetable Centre. In such a case, you'll have to check timetables on the operator's own website. That's a good idea anyway since the rail planner app doesn't get enough updates to be reliable.

You can find Irish timetables here:

Irish Rail (Republic of Ireland)

Translink (Northern Ireland, includes all public transport)

Perfect! Thank you very much for the reply. How do I find out, whether I need reservations or not? For instance for a ferry from Scottland to Ireland or Ireland to France and how much the supplement costs?

Reservations on trains in Ireland are not required. Check other country pages here.

For ferries, you can check the ferry page. Both Irish Ferries and Stena Line offer a 30% discount on the foot passenger fare (accommodation is extra). For booking instructions, check the pages mentioned. You will need a reservation indeed.

Since you only get a discount, you don't need a travel day if you don't use a train on the same day.

Hint: here were (are? check, may be cancelled due to covid now) direct BUSes (coach for /en/) from Glasgow to Ulster, via the ferry and these were often even cheaper as the single ferry-fare for foot-pax.

Also note that several ferries have stopped taking single foot-pax too or there is a very cumbersome or sometimes non-existent link from berth/seaside to nearest bus/trainstation. F.e. the old Stranraer port is not used anymore in Scotland (end of the railline, side byside to ferry then)