I've done similar one morning. I was sure the driver was either drunk or suicidal, e.g. Only overtaking when approaching a corner, overtaking a line of cars when there were hills which prevent seeing any cars coming towards his direction of travel. I phoned Garda station and gave registration and direction. Ten minutes later, Gardai had set up roadblock. Driver was stopped, charged with dangerous driving ( Garda saw driver overtaking over double white line at speed). I was asked to give statement later that week which I did but court appearance was not necessary as driver pleaded guilty.