Hello Shieloafaigh39
Regular train services offer free seating. We use our trains as others use buses in their countries: hop on, find an empty seat, have a ride, hop off. Nobody here reserves seats on regular train services, and if, on very busy lines.
If you travel with a group of more than 10 people, you can do group reservations upfront. In this case, you will have your seats reserved in a specific train compartment. This works on IR and IC trains services.
There is actually no need to reserve seats as individuals, as our trains are rarely very busy, especially during the day. During the peak hours, you may have to stand in the aisle for one or two stations. If a train is fully crowded? You simply take the next. Also, it is advisable to board the trains at the front or at the rear, not in the centre where most people get on. The seat load factor is visible in the timetable by the three human icons. Three red human icons mean, the train is usually busy at this time, three white human icons mean, the train is almost empty at this time.
Kind regards,
Roland