There's another point that hasn't been made here and that I'm afraid I can't give any sources to, partly because it's a homemade theory.

But in general, nationalism, or rather, the construction of the nation, is connected to the construction of the other. So we as a nation are whatever they are not.

Something that separates Ireland from a lot of other European countries is it's self-image as a former colony, a formerly oppressed country. Irish nationalism (and nationality) is constructed as the anti-thesis of the oppressor, namely the english. In their collective memory, the national struggle coincided with the class struggle, and the british occupation of Northern Ireland keeps that self-image alive.

This happens in a lot of former colonies. As you'll see in South America, Africa and South-East Asia, nationalism is closely knitted with "leftism". In many cases, the national (independence) struggle was even described in marxist terms (The wretched of the earth by Franz Fanon is a great example of this).

Here I wan't to explain something more, South America was not liberated in the middle of the 20th century like Africa, but a lot of the current governments in South America come from the "second" struggle, which was against US-supported military dictatures. In many cases in south america, the national identity was reborn around the middle of the 20th century, now constructed as the opposite of American capitalism-imperialism instead of European imperialism. And American capitalism in South America keeps that self-image alive.

In Europe on the other hand, there wasn't the construction of the other as a foreign oppressive power. Some countries have had that, and constructed (or reconstructed) their identity after the world wars. Denmark, Norway, Poland and Yugoslavia from the top of my head are all countries that have a very vivid memory of the war and have constructed their national identity after it.

But during the last decades, as there no longer is (or has never been) a foreign oppressor for the European countries, and because of this, in rough times, there has been a need to formulate a new other - immigrants/muslims.

I don't know if I've made myself clear but the TL;DR would be: Irelands national self image is constructed more like the one of former colonies, and less like the one of European powers. This is why nationalism is left wing in colonies and right wing in former imperialist (or European) countries.