Mother Jones has a very interesting series of articles at the moment on how lead air pollution - mainly from leaded petrol - was a major cause of high crime rates seen during the late 20th century. It sounds extraordinary, but lead damages the brain development of children and has been shown to make them grow into more violent and impulsive adults, and more prone to crime. This particularly effects disadvantaged kids who grow up in the inner cities - where there was substantially higher levels of lead pollution in the air.
America's Real Criminal Element: Lead | Mother Jones
The article above is definitely worth a read. Its main suggestion is that crime increase about 20 after high lead air pollution is seen - because it takes 20 years for the effected kids to grow up and commit crimes - drops around 20 years after lead pollution is reduced.
Similar numbers are repeated in every country that has been studied: http://pic.plover.com/Nevin/Nevin2007.pdf
Ireland didn't ban leaded petrol until 2000, a number of years after the US. I looked up the Irish EPA' lead pollution records which are available online back to 1997. In that year the concentration of lead in the air of Dublin was up to 400 ng/m^3. By 2002, (after the ban) it had dropped to 20 ng/m^2. By 2012 this had dropped to around 3 ng/m^3 - which is more than 100 times less than in 1997.
With a 20 year lag, we can expect to see crime rates begin to fall over the next few years. If the US example is repeated here, violent crime may fall by up to 80% by the year 2020.