Just over €1 billion was collected in motor tax in 2015, with two-thirds collected in online payments.
The research, which comes from the Office of the Comptroller and Auditor General and the Local Government Audit Service, also shows motor tax revenue has fallen despite an increase in the number of taxed vehicles.
This is largely due to the fact that significantly lower tax is paid on newer private cars on the basis of engine emissions, compared to those taxed on the basis of engine size.
The report suggests this will lead to an annual fall in revenue of around €260m by 2024.
The cost of administering the motor tax payment service is nearly €50m annually.
At present, the level of motor tax compliance is not monitored.
Analysis of vehicles on the M50 in 2010 and 2011 suggested an evasion rate of around 5%, however, such analysis has not been repeated since due to data protection concerns.
In the UK, motor tax evasion rose from 0.6% to 1.4% when the country moved away from paper tax discs.
A review of motor tax transactions suggested an increase in the proportion of frequent transfers of vehicle ownership.
This practice may be used to evade motor tax, as arrears or not pursued when vehicle ownership changes.
Though the examination indicates possible evasion rates with regard to off-the-road declarations are down since changes to the system in October 2013.
Prior to that date, retrospective off-the-road declarations could be made – with nearly 600,000 such declarations in 2011.
However, from October 2013 legislation came into effect requiring a declaration to be made in advance that a vehicle will not be used on public roads.
This saw the number of off-the-road declarations fall to 104,000 in 2014.
