A number of explanations for the colourful doors have been offered. One widespread but apparently apocryphal story is that Queen Victoria ordered all the doors of the British Empire to be painted black, in mourning for Prince Albert. The Irish reacted by painting them in rainbow colours. The most likely explanation is that the Georgian style exteriors were designed to be very uniform, down to the smallest detail, so painting the doors in different colours enabled residents to set themselves apart.
The Georgian doors have been photographed many times and have their own poster and Facebook group.