RTÉ One

Logical Channel Numbers are assigned by the platform, not carried in the transmission, which is why they differ between nations and why a retune can move them.
Channel number	Where that number applies	Multiplex	System	Format	Availability
54	Northern Ireland	NI (NI mux)	DVB-T	16:9 SDTV

On NI (NI mux)

8 transmitters radiate NI [V] — a small number of sites. By nation: Northern Ireland (8). It is operated by Northern Ireland Multiplex Ltd.

Every transmitter Ofcom publish as radiating NI.
Transmitter	Nation	Channel	ERP
Armagh	Northern Ireland	C48	24 W
Black Mountain	Northern Ireland	C33	3.2 kW
Brougher Mountain	Northern Ireland	C30	1.0 kW
Carnmoney Hill	Northern Ireland	C48	16 W
Divis	Northern Ireland	C48	10 W
Newcastle	Northern Ireland	C48	200 W
Strabane	Northern Ireland	C40	20 W
Whitehead	Northern Ireland	C48	2 W

If this is missing, so is everything here

3 other services share NI. A multiplex is one 8 MHz transmission carrying all of them at once, so it arrives whole or not at all — losing exactly one of these and keeping the rest is a fault in the channel, not in your reception.

TG4RTÉ2RTÉ Raidió na Gaeltachta

Which multiplexes should reach a particular address is the postcode lookup.