In Ireland all “Intercity” designated trains can be booked online and seat reserved.

Seat reservations can be made for free at https://journeyplanner.irishrail.ie/webapp

use the “free travel or valid ticket holders” option and available trains will appear for €0.

There is a seat selector available so you can choose your seat.

If your friend has not already bought a ticket then they can buy theirs online and add a second person as above for free seat reservation and do both their bought ticket with res and your free res together.

If they already have a ticket then take the seat No and assign yourself one beside it.