Certainly every older house now has one. (It can take a few months, and there's currently a delay, so some newbuilds may not yet have one. I believe the advice is to apply well before the house is finished.) How much they're used varies. One house near me has a plate on the gate giving the name and eircode. I think that most people use them most of the time. I don't know whether there are any stats available on that.
Of course, if you've been subscribed to a monthly magazine for fifty years, you perhaps haven't bothered to update your address on file to add the eircode.
Of course, if you've been subscribed to a monthly magazine for fifty years, you perhaps haven't bothered to update your address on file to add the eircode.