Current plates have black characters on reflective white PVC (for the front plate) or on reflective yellow PVC(for the rear plate). These are the colours used in the common EU plate format (though not all other EU states use the two background colours in this same way). Older plates had white, grey or silver characters on a black background. This style of plate was phased out in 1972, and is now legal to be carried only on vehicles built prior to 1 January 1973.

Front number plates are 520mm x 111mm (20.5" x 4 3/8")in size. Rear plates may be 520mm x 111mm (Standard), 285mm x 203mm (Square), 533mm x 152mm (large).

The current system in use in Northern Ireland is administered by Driver and Vehicle Licensing Northern Ireland (DVLNI). Current registrations plates take the form "ABC 1000", where "BC" represents the county or city and "A" denotes the position in the series. The numbering begins at 1 and ends at 9999. After 9999, the next letter in alphabetical order is used at position "A" and the numbering series begins with 1000 once again. For example, in 2005 County Antrim is using the series "KZ", having already completed "IA" and "DZ". After "KZ" is exhausted, it will use "RZ". The full list of county codes appears below.

The county letters without the series position identifier were used previously on their own, in the same order that they are now being used. After all these registrations had been issued, the extra letter was added to increase capacity. Numbers below 1000 are now not issued to the public in the normal way but instead held back by DVLNI and supplied at a premium as vanity plates.

This system was also used in the Republic of Ireland until 1987 as part of an original British all-Ireland system. It was similar to an older system used in Great Britain, but the use of the letters I and Z is unique to Ireland. In this system, two-letter county codes existed for all counties or administrative areas in Ireland, but are now used only in Northern Ireland. (See also: Vehicle Registration Plates, Republic of Ireland)

The DVLNI are considering adopting the new system used in the rest of the UK, using I as the first letter (no confusion could be made with 1 as it would be followed by another letter). Northern Ireland licence plates are used often in Great Britain as vanity plates to cheaply hide the age of an older vehicle.

The County Fermanagh registrations KIL, CIG and NIG were deemed inappropriate and will never be issued.

The European Union standard for number plates causes some degree of resentment in Northern Ireland as the internationally recognised number-plate code for all of the United Kingdom is 'GB' for Great Britain, which can appear to exclude Northern Ireland. For political reasons, some people may choose to use an 'IRL' version, e.g. |IRL| ACZ 0000|, although this is incorrect in terms of the European numbering system and is illegal according to UK number plate regulations. There are Northern Ireland registration plates which, quite unofficially, use the EU style blue strip on the left hand side with no country code written; ie the blue strip just shows the European stars. Another unofficial code is "NI".