The legality is that you can drive a foreign car in Ireland for no longer than 30 days. After 30 days it has to comply with all Irish rules: VRT (if you cannot prove you are an owner for more than 6 months), registration, NCT and insurance.
Please have a read here, it is well explained:
http://www.citizensinformation.ie/en/moving_country/moving_to_ireland/coming_to_live_in_ireland/importing_car_into_ireland.html
If I were you, I would drive Polish car, if it is only for 3 summer months. You won't be stopped that much. But always keep your arrival ferry ticket in your glovebox and leaving ticket if possible. Guarda is fair when it comes to these situations, if you can prove you stay here only temporary. If it is a flashy car, it may be a bit harder as local guarda tends to remember you
I went through this. Managed to stay with Lithuanian reg for 9 months, but then guarda was nagging me everytime they stopped me, threatening to confiscate the car and etc., which they are entitled to do.