Think UK "custom" numberplates are limited?
The Irish (Republic) system is even more restrictive -
The standard for regs is YY-C-NNNN, where YY is the year ie 07, C is the county e.g. D=Dublin, C=Cork etc, NNNN is a sequential number eg 34704. Nice and simple, the age of the car is easy to pick out, and where it's from.
One problem with this is that while at the start of the year all is fine eg. 07-D-123 is nice and easy to read, towards the end of the year the numbers get longer eg. 07-D-110573. Try reading the numerical part of the reg after the car does a hit and run. Not too easy. And think of all the regs that it could look like if you're out by a digit!
Right, back to the matter in hand - customising - you can't. Even down to the format of the plate and the font itself. Changing the font is an offence (afaik), and will certainly fail your NCT (MOT equiv) if you do.
The "creative" types try to do things with the number part. So every single Porsche in the country has some variation of YY-D-911. Well done. Its a 911. We know. (This technique also works for Peugeots, Rovers, BMW etc)
Having said all this, I do think that in Cork, in 2036 or Dublin in the same year, there will be sniggers as all the regs start with either 36-C or 36-D and a few lucky punters will get their grubby mits on the 7175th car, or even the 80085th car in their county. (Squinting may help, also having a juevenile mind!)