While there is no physical border between Ireland and Northern Ireland and no passport or customs checks, there is an invisible border. You will know you have crossed the border, predominantly by the road signs. Road signs change when you cross from Ireland into Northern Ireland. Speed limits and distance signs in Northern Ireland are shown in miles, while they are in kilometres in the Republic of Ireland. You might see signs saying “Welcome to Northern Ireland/town in NI” or “You are now entering NI/town in NI”.