I could write a whole essay on this - being a personally researched area of mine.
There were surprisingly few Irish who fought for the Nazis at all - or even helped them. Tens of thousands fought for Britain and the US. It's not even hundreds for the Nazis.
The reasons are complicated. The main reason for not fighting for the Nazis was Hitler and Catholicism and the Pope. It was not some kind of sense of Nazi atrocities. Hitler used to hassle the Catholic church and the Irish were well aware of it. The Pope denounced Nazism as a cult - and that was enough for most Irish.
Ireland loved Mussolini - for similar reasons - gave the Pope the Vatican etc.
A lot of Irish were recruited from prisoners from the BEF. Many were in Nazi special forces as English speakers. The guys you see with Skorzeny in The Battle of the Bulge movie would have likely been Irish recruited that way. My own personal feeling is that you can't be too harsh on someone who gets out of prison that way - when after all, there was no particular love or loyalty to Britain.
Skorzeny and many others ended up in Ireland, shielded by the government - Ireland should be more ashamed of that and their turning away of the Jews at the beginning of the war, than anything during. In fact, Ireland used to drive downed British aircrew to NI in covered trucks. They kept the Germans in the Curragh military camp. Remember, Dublin was bombed "accidentally" as it were.