This is a Republic of Ireland state railways Iarnród Éireann diesel multiple unit of the Irish class 22000 visiting North Ireland. North Ireland (Ulster) has its own Northern Ireland Railways NIR, now operated in passenger traffic by Translink, which also operates most of the bus traffic in Northern Ireland. But these border crossing Intercity services between Dublin Connolly station in the Republic of Ireland and Belfast in Ulster are operated by specifically modified Iarnród Éireann trains under a contract between Iarnród Éireann and NIR.

Although Northern Ireland and the Republic of Ireland share the same track gauge width of 1600 mm, the train security systems are totally different. Northern Ireland uses systems from mainland UK whereas Ireland has its own totally different systems. Only 10 trains of the diesel multiple unit type class 22000 of Iarnród Éireann are today fitted with both systems so that they can cross the border. Northern Ireland today has only two heavy diesel locomotives which are similar with Iarnród Éireann class 201 and these machines can cross the border. But almost no other trains can.
Picture of an Iarnród Éireann class 22000 diesel multiple unit Intercity train at Belfast Grand Central Station 5.6.2025 by Ilkka Siissalo.

Northern Ireland Railways is a subsidiary of the company Translink, which also runs most of the bus traffic in NI. There are five rail routes and today all passenger traffic is operated with similar looking trains which are either three coaches or six coaches long. This train is one of the six coaches long sets. These are called class 4000 and they were delivered from CAF, Spain 2010-2012. There are 20 of these class 4000 trains.
Picture from York street station in Belfast 4.6.2025 by Ilkka Siissalo.

Same train a couple of seconds later. Six wagons may not sound like a lot, but the CAF made wagons are really long, so a class 4000 train is actually impressively long as a unit.
Picture from York street station in Belfast 4.6.2025 by Ilkka Siissalo.

A closer look at the front part of a class 4000 train.
Picture from York street station in Belfast 4.6.2025 by Ilkka Siissalo.

The same class 4000 train as shown in the pictures above was still staying at the station as a three coaches long class 3000 train approached from the opposite direction. At first glance the class 3000 and class 4000 trains may look like exactly the same, but if you look more closely you can notice that the yellow paintings at the trains fronts are different and the class 3000 has rectangular headlights. Both train types have been built by CAF but the three coaches long class 3000 is somewhat older. They were delivered 2003-05. There are 23 of these shorter trains in Northern Ireland.
Picture from York street station in Belfast 4.6.2025 by Ilkka Siissalo.

A class 3000 train, three coaches long.
Picture from Belfast Grand Central station 5.6.2025 by Ilkka Siissalo.