I am not surprised that the discounters Aldi and Lidl have just 6% of the market.
Irish people are generally brand conscious and quite brand-loyal. Many will be sceptical of the unknown or private label brands in the German discount stores.
Next, the Irish are not as price sensitive as other Europeans, so will be slower to move to discount stores.
Also, I'd say there is still a bit of "snobbery" associated, i.e. sort of "only poor people and "refugees" go there"
In particular, I'd say people will be slow to buy certain products from there, e.g. fresh meat