Using Irish ATM

This is one of those urban myths that refuses to go away, because you can't prove a negative.

First FACT. European banks (including mine) issue 6-digit PINs to their customers and these work all over Europe.
Second, where are you from, and where is your bank from (which country)?

Cards in some countries are different to those in other countries. For example most of Europe uses chip-and-pin, the USA doesn't. Unmanned ticket machines often require chip-and-pin, so US cards don't work.

So I cannot definitively say there isn't some bank in your country which issues cards which are in some way incompatible with Irish machines and only work with 4-digits (or they may only work with 6-digits).
Sorry if that is not a very definitive answer, but it is all you will get, unless you try it yourself.

not true. I have a 4 digit pin and was just fine

Sharon, what do you mean by "not true. I have a 4 digit pin and was just fine"?

Just because your 4-digit PIN works (I also have one card with a 4-digit PIN) doesn't mean that 6-digit PINs don't work.
This may also have been an historical problem, 10 or 20 years ago but not now.
All I am saying is I have never seen anybody state definitively that they tried a 6-digit PIN and it didn't work in any machine, and my 6-digit PIN DOES work, it is all rumour.