In practice what happens is that very often the MPs more or less compete for constituency work. If they get any hint of a problem, such as a water main burst or inadequate schools in some area, every TD from the constituency wants to get involved and wants to be seen as the person who fixed it.
Constituents don't face a problem of MPs shirking and saying “nothing to do with me”. Quite the contrary; every MP wants the constituency work, because they all feel that this is a way to build personal support. Irish constituents certainly don't feel that their TDs neglect them. As I said before, commentators say they shouldn't be doing this, which is a sign, really, of how much they do.
So PR-STV certainly doesn't lead to constituents not having any TD to take a problem to.