Been involved in two with different firms one where I was directly responsible for tax paperwork one where I facilitated the audit only.
Both times the Revenue officials were thorough, polite and professional.
Look at Revenue .ie for FAQ on audits.
If you suspect you have not been exact with your returns you can 'fess up before audit starts and save penalties (but not dues) if the errors are deemed as honest mistakes as opposed to avoidance.
They say and I believe that they only want to collect appropriate tax due.
If it is obvious that you are compliant they won't waste your time or their's.
If it is obvious that you are not compliant they can go back further than originally notified...