All immigration offices in country are closed tomorrow the 14th due to a public holiday, they will be open again on Tuesday the 15th. Of course border checkpoints and airport immigration will be open as normal.
It is totally up to the immigration office you are using, some will require it, some will not, you are always best to go and ask what your particular office will require from you.
Yes Irish passport holders are eligible for a 60 day visa exempt stamp upon entering Thailand, this can be extended one time for a further 30 days at your local immigration office for 1,900 baht. The 60 day visa exempt stamp is free of charge.