Try Longstone in the UK maybe?
http://www.longstonetyres.co.uk/page/index (site seems to have some issues right now though).
Z or even ZR rated ones will be hard to find though.
For sure Pirelli, Falken, Hankook, and Fulda still have 225/50/15.
Can't tell for the 275 rear ones - I use 295. Don't know how things work in Ireland, but not all of the US tires are allowed to be used in Belgium.
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