That the actual speed limit could be (as in the case of that picture, chosen from a large set of possible candidates) double what the reasonable driving speed might be is, in my experience, more unique to Ireland than anywhere else. And after fifteen minutes driving the only reason it's still surprising is that the signs are there.
As you say, the surprising thing is the signs. Every European country has a general speed limit and has a vast number of small roads where nobody has ever suggested that this limit is appropriate, but where they also haven't wasted time posting a lot of different limits. Present signposting in Ireland, which dates from 2005, draws your attention to the general limit. A curiosity of signposting, but entirely immaterial to driving conditions or their difficulty. Where specific lower limits are posted in Ireland some of these are too low also.