Fewer than half of the drink-driving prosecutions initiated by gardaí last year resulted in a conviction, according to an RTÉ investigation.

Convictions were recorded in only 58% of cases brought before district courts last year, but summonses were not even served in almost another 1,000 cases, resulting in an overall conviction rate of 48%, an RTÉ Investigates programme tonight will show.

A further 800 drink-driving cases were "adjourned, struck out or dismissed" during 2014 and 2015 as a result of a successful High Court challenge by a man from Romania who was not given a copy of his breathalyser reading in the Irish language.

The programme, RTÉ Investigates: Law and Disorder, to be broadcast on RTÉ One at 9.35pm tonight, will also show that district court judges dealt with more than 800 penalty point cases using the court poor box in 2015 even though the High Court had ruled a year earlier that to do so was to circumvent the law.

Courts Service figures provided to RTÉ show that in 2016 the average case required 2.5 court appearances. 

One case in Kilrush was adjourned 41 times before reaching a conclusion on the 42nd appearance. Other cases in Killarney with 39 adjournments, Dundalk, 35, and Sligo, 35, took up to five years before concluding in 2016.

Many of those cases required the services of the free legal aid scheme which cost the taxpayer €53m in 2016.

The programme will also show that almost 2,000 crimes per month, on average, were committed by people on bail over the past decade.