Only 27% of motorists in Mayo, who were charged with using a mobile phone while driving, have been convicted of the offence over the past two and a half years.
After Co Clare, Mayo had the lowest conviction rate at 27.1% - compared to a conviction rate of almost 72% in Co Longford.
The figures were released by the Courts Service to Independent TD Tommy Broughan, covering the period from January 2017 to May this year.
The figures show one-quarter of motorists who were supposed to appear in court for using a mobile phone while driving have been let off, because they never received a summons.
Of those who did go before a judge, only half were convicted.
Some of the cases were dismissed because prosecuting Gardai did not turn up in court, while others were thrown out because the wrong person was prosecuted.
The Parc Road Safety Group says the 50% conviction rate nationally is disappointing, and sends out a worrying message about the state of road traffic law enforcement in this country.