Uk - Irish Citizen (Cork)

Hi All, looking for some advice. I am British born and moved to Ireland permenantly 10 years ago when I was 25, I have been working here and paying tax here for this entire time. I am moving back to the UK next month for good. Earlier this year I bought a UK reg car back in the UK and I brought it back to Ireland in August for my last few months here. Last week I relieved a random visit from customs and excise who have now fined me €400 for not having paid VRT on the car and having registered it in Ireland. I explained my circumstances but the customs officer told me that that this is irrelevant as I am automatically an Irish Citizen having lived here for over 6 years. I do not believe this to be the case. I am not married nor ever have been and I have never applied for or been granted Irish Citizenship. I am unable to find any clear and definitive advice on this online. Can anyone help clarify re my citizenship status and where the revenue right in stating that I am obliged to register the car here despite the fact that I will very shortly be returning to the UK for good?
I found the below information on gov.uk/living-in-Ireland

Importing a car into Ireland
All cars/vehicles brought or imported into Ireland (apart from temporary visits of less than 12 months) are subject to Vehicle Registration Tax and must be registered with the Revenue Commissioners. The tax rates vary depending on engine size – but are around 25% of the expected selling price.