But it's not. And with the introduction of Metro North, it stands to get even less unified.
Also, we had a unified brand for transport up to 1984, it was called CIE.
The reason you see so many livery and brand changes in Ireland is because the services are generally diabolical so they create flash new brands to paper over the cracks and try to convince people that "this time it'll be different". Think "City Swift" with their stupid named bus stops where the name of the stop could be different depending on which side of the road it was on
In Berlin the underground rolling stock, buses and trams have been plain yellow for about 80 years now. Sometimes they change the shade (maybe once every 20 years+).