Unless you have specifically told your hire car company that you will be crossing the border, then no. As they are two separate countries, the hire car companies need to ensure the insurance premium covers driving in both countries and this can add significant cost to your rental car budget. And don’t think you can just slip through unnoticed. If the car is fitted with a tracker, the rental car company will be notified and you will be charged a premium for crossing the border without telling them beforehand.