Is there a minimum time you need to be resident in the UK before you buy a car? Someone told me you needed to be there 3-6 months....but I've never heard this before, I presume it's rubbish?
I presume you are referring to then bringing the car back to Ireland Kine? If so then the legislation is that you must pay VRT on the car when bringing it here unless you have lived in the UK for a minimum of 6 months after you bought the car.
You can buy a car whenever you like, but if you want to avail of the VRT exemption it is only for a genuine transfer of residence to Ireland and you have to have extensive documentation to prove this.
OP - there are loads of posts on this topic, including the VAT/VRT/export/registration aspects etc.