Assuming that you hold a Thai passport and no other nationality, you can transit without a visa as long as you do not change terminals OR have checked baggage.
Your flight details should tell you what terminal your flights land/depart from, and whether they are different.
The second bit on no checked bags also applies to gate-checked bags for larger items of luggage, which it will in most cases. The only way to avoid having this gate-checked (and therefore forced to go through border control to baggage claim) will be to buy Priority Boarding.
This is from INIS:
You do not need a transit visa*
- You do not need a transit visa, subject to the information below, to travel through an Irish airport/seaport on your way to another
destination. When travelling
To transit through Ireland means you are only passing through an Irish
airport or seaport to travel to another destination. To transit, you
cannot pass through border control. If you pass through border
control, you are not transiting.
As you are visa required to travel to Ireland but your nationality is
not listed on the transit visa required list, you must check with your
airline to see if you have to pass through border control.
If you have to pass through border control, you need a short stay or
long stay visa to travel to Ireland. When do I pass through border
control?
Times you would pass through border control would include, but are not
limited to:
- Leaving the terminal building for any purpose, including in some cases having to transfer between terminals for a connecting flight.
- In cases where you must collect your bags and check them in to your onward flight.
- Transiting overnight - all Irish airports close at night and passengers are required to pass through border control.
Before you travel please check with your Airline if you have to pass
through border control.
