Colin McKenzie wrote to uk.transport.london on Mon, 3 Jan 2005:

I think it was for compatibility with the rest of Ireland, though it
continued after the Republic went over to a new system in the late '80s.
I think they use the same system as the rest of the UK now.

Not as far as I know - I think they are still on 3 letters/4 numbers.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 2 January 2005