This page: https://48.ie/charges/other-call-charges 

claims to support short code messaging.
Unfortunately it's not (fully) true.

According to my chat with support today, only a very select list of short codes are actually supported:
50300 0c
57802 0c
57886 10c
57557 10c
57765 10c
57666 11c
53125 60c
57117 200c
50300 0c
53125 0c
57117 0c
57666 100c
57765 200c
57802 200c
57886 250c
57557 ?
50300 0c
57802 0c
57886 10c
57557 10c
57765 10c Y
57666 11c Y
53125 60c Y
57117 200c
50300 0c
53125 0c
57117 0c
57666 100c
57765 200c
57802 200c
57886 250c
57557 250c

To me it's a bit bizarre.
The numbers in the free and lower cost ranges are all the same price within the band, and only numbers above 57000 need individual pricing.

Why the limitation?
Nobody knows... 

Time to shop around perhaps.