That's not true (as somebody who had to do this last year in 2024 because I was traveling in another country for work on election day)
Here is the form to register for postal voting in the Republic of Ireland - https://www.dublincity.ie/sites/default/files/2024-01/pv4-wo...
Instructions on how to submit the form / register for mail-in votes is on page 4.
Hope that helps anyone else out who needs in Ireland
I think they meant "don't have it" as in except in special circumstances, and that form says:
> You may use this form to apply for a postal vote if, due to the circumstances of your work/service or your full-time study in the State, you cannot go to your polling station on polling day.
Which seems to indicate that's only for people who can't go to the polling station, otherwise you do have to go there.