I used the redirect 2 different times.
Firstly, about 6 years ago, and it was very hit and miss. I complained to An Post, and they said the quality of the service depended on the individual postman on the route, as they had a list of redirects and it was them who returned the post to be redirected. I lived in city centre where the postman changed frequently / temporarily, and also I can't imagine it was easy to remember individual apartments.
Used the service again this year, and it seems to have been better. Don't know if the method has changed, or if it was because we had a regular postman. I didn't have access to the house this time to confirm.