What Americans call “Irish Bacon” would in Ireland be referred to something like “traditional back bacon.” It’s cured (e.g. brined) pork, often cut from the eye of loin. The fat isn’t trimmed off the edges, as it is in what Americans call “Canadian” bacon (aka back bacon in Canada.) It may be smoked or unsmoked.
Americans may also use the term Irish Bacon to describe a whole raw piece of bacon — as in, one that is brined, but not yet sliced up — that is cooked whole, as in the Irish dish of “cabbage and bacon.”
Irish Bacon is the same as bacon in the UK.