Etiquette for academic job interviews in Ireland
I am a USA academic, raised in the UK, who interviewed unsuccessfully in Ireland. I was partially unsuccessful as I realized a few hours in that I did not want to work there but as they had flown me over I did not want to be unpolite and leave the interview.

It is not, you may contact the head of the search but not anyone else.
Yes, both male candidates wore suits and ties and both female candidates wore skirts/pants with a shirt and jacket but neither wore a tie.
The most difficult, you could bring this up if you get to ask questions. I would not make it your first question and I would be brief. Does the university offer startup packages?

What was most different than in the US was.

I got to meet all the other applicants. We were in the same room and called one-by-one to do our presentations. I ended up really wanting another candidate to get the job.
Only one or two of the people on the panel was a faculty member in the department.
No dinner with faculty, not a tour of the town, no real contact with the department at all aside from the odd quick hello as we passed their offices.