Always good to see this sort of thing but an odd train from, err, 'not Armagh' to 'not Croke Park' in Dublin isn't very different from what GB often manages for a major sports event.
It's not about Armagh the city, but Armagh the county - since it's to transport supporters of the county's Gaelic Football team. Both Portadown and (parts of) Newry are in County Armagh, plus they act as rail heads for the rest of the County.
Connolly Station is easily walkable from Croke Park.
Providing additional services for events at Cardiff, Glastonbury (even when there is major industrial action going on), Wembley, etc. is pretty much 'bread and butter' stuff. Not knocking the Irish train, though; it's good that they can do this, too.
The difference between Ireland (North and South) and Great Britain is that services like these are a normal and expected part of the service provision of the companies involved (NIR and IÉ). They've been running these services for big matches and other events for decades, and have a fleet large enough to cope. This particular special will almost certainly be worked by a number of NIR 3000 class DMUs - and when that fleet was procured,a requirement was that part of the fleet would be compatible for working South of the border, to work services like this (and of course to cover for unavailable Enterprise sets).
In the UK, most TOCs are not in a position to provide a similar scale of extra services on a regular basis.
It's not so much going from Derry to Cork more people getting a service between two intermediate stops. For example on the CrossCountry services very few people travel from Plymouth to Edinburgh but rather between intermediate stops on the route such as Bristol to Leeds.
The numbers making the intermediate journeys are even fewer. And the complications of running search a service are much more complicated than what XC have to deal with. For a train to cross the border, it needs to be fitted with both countries signalling systems (AWS & TPWS for the North, CAWS and soon the new hybrid ETCS system for the South) and staff need the training for those systems, plus the other minor differences between the two jurisdictions. It's worth the hassle to link the two largest cities on the island, and enough staff and trains are in place to deal with it, but beyond that.....