The DUI rules have tightened a very great deal since this song was recorded. These days you'd go to jail for a long time just for doing a hundred, if they caught you. If they did.
I understand Ireland has repealed DUI laws for the countryside, and only enforces them in the city. They decided it was better, and less of a risk to everyone overall, to let country people drink in pubs together than to make them drink alone at home. Probably less of a risk to let them drive a hundred on country backroads, too, than to chase them down.