Quark, or tvorog or topfen or biezpiens or varškė, is a medium solid milk product.
Quark as Germans are used to is not well known in Ireland. Thus it is hard to find it. It is typically used for cheese cake.
Various larger Polish or Lituanian shops on Talbot St or in the city centre have quark in 1kg buckets for at least €5.
You should get quark in Sainsbury's in Northern Ireland.
Superquinn in Blanchardstown Village.