Hi Adrian,

There isn't any €370 tax rate that we know of so we assume you mean €390. 

Assuming the cars otherwise appear identical, with the same sized wheels and gearbox, then it may be down to one being imported from the UK since the rules changed on 1 Jan 2021. It may have had its emissions rating 'uplifted' to a WLTP-equivalent figure, putting it in the 141-150g/km bracket, which is €270 per annum for cars registered in Ireland on or after that date. The other car is presumably an original Irish car, with emissions of 121-130g/km on the old NEDC system, which, weirdly I know, cost more to tax - €370 per year.