The best place to find authentic Georgian doors, now painted, is the south side of Dublin
city centre, where the 1700s architecture still remains.Merrion Square
, Fitzwilliam Square
, Lower Baggot Street Lower
and St Stephen's Green
are all good places to look...so you'll be based in the right place!
Please don't be misled by the urban legends too often promulgated by tour guides and media. The doors weren't painted 'by women to stop their drunk husbands going into the wrong house' (obviously!), nor were they painted because there was an order to paint them black when Queen Victoria died and the Irish refused (there was *no* such order) and it's also pretty unlikely that George Moore's dispute with his neighbours about his green door led to it becoming the fashion.
The reason why they were painted in different colours is much simpler. Georgian town architecture results in the exterior of all the houses in a section looking the same, and quite plain. Painting the front door a specific colour was (and is) the only obvious way to distinguish one's house from the others in the row. You'll see exactly the same in other European cities with similar architecture e.g. London.
Do look at the fanlights (the window above the door). They, unlike the door colour, are likely to be original and they vary widely (the whole idea of 'Dublin doors' has only been around since the 1970s). Having a specific fanlight pattern helped a largely illiterate working population to find a particular address (e.g. for delivery purposes): you'll see the same variation in fanlights in other European locations such as London and Amsterdam.