Hi Siim
There are a couple of  model shops in Dublin.
The first one is on Blessington Street just at the junction of Dorset Street. It quite a small shop but usually well stocked with a varied selection of plastic kits. As much a meeting venue as a model shop.  
The other one is Mark's Models on Hawkins street.
Mostly model railway stuff but with a good cross section of plastic kits.
Both are within easy walking distance of O'Connell Street(main street of Dublin)
There is also a couple of regular toy shops on Jervis Street( Banba toys & Smyths toys) which some times carry a stock of kits, but they can be a hit or miss as to choice and it would be mostly older kits from Airfix, Tamiya or Revell. 
I have not been in Galway in years and have no idea of any model shops there, maybe someone from the west can advise further. 
I hope this is of help and that you enjoy your trip to Ireland
Regards
Vincent 
