The Road Transport (Vehicle Registration) Regulation 2007 stipulates that a registration label must be affixed on the inside of the lower-left portion of the front windscreen or on a fixed glass panel on the left-hand side of a vehicle.
For caravans and other trailers with a rear facing window, the label may be stuck on the inside of that window in a position as close as possible to the number plate. Hinged windows displaying a rego label must be closed while the vehicle is being towed.
For any other vehicles, such as motorcycles, the label should be affixed on or adjacent to the vehicle's rear number plate without obscuring the characters on the number plate.
The law also requires that a registration label 'must be affixed so that the information on the label is readable from the outside of the vehicle'.
A tubular registration label holder can be used for motorcycles if it meets the above requirements.
Information supplied by the Roads and Traffic Authority - September 2008.