From Timatic for Irish citizens:
Passport required.
- Passport and/or passport replacing documents must be valid
at least 6 months after the date of arrival.
Visa required.
Visa Issuance:
- On arrival: if required, a Visitor's Pass is issued to
nationals of Ireland (Republic of).
Additional Information:
- Visitors must hold return/onward tickets and all documents
required for their next destination.
Warning:
- Non-compliance with visa requirements will result in
deportation of passenger on same flight.
Appears that Irish passport holders are exempt from the $50 fee, many UK citizens are also entitled to Irish citizenship, so if you plan on making a few trips then it might be worthwhile finding out if you're eligible. Normal requirement is an Irish born (North or South) parent or grandparent.