Cleaning-up after vandals on the Luas costs an average of €1m a year, according to new figures.
And new security guards have been hired to clamp down on vandalism and anti-social behaviour on the capital's tram system.
Transport Minister Leo Varadkar revealed the cost in a reply to a Dail question. It showed this year recording the highest annual cost for cleaning-up trams after vandals, with a total spend of almost €1.4m for the first nine months of the year.
The vandalism includes graffiti, litter, damage to tram cars and anti-social behaviour on the Luas.
The costs worked out at a total of €5.2m between 2009 and the first nine months of this year, and averaged out at around €1m a year.
Mr Varadkar said no figures are available for how much was spent on security on the Luas.
"A number of measures are in place to tackle anti-social behaviour and vandalism on Luas, including extra security staff and revenue protection officers," Mr Varadkar said.
But he could not reveal the security costs, because that was a confidential commercial deal with a private company, Transdev.
Other measures being used to tackle vandalism and anti-social behaviour include CCTV, closer co-operation with gardai, prosecuting offenders and working with local communities in the worst affected areas.