The form of ID you'll need to travel on a ferry depends on the route and your own nationality. On most international routes you will need your passport to check-in, however there are exceptions to this. If you're an EU citizen, travelling in the EU, a valid EU national identity card can also be used. When sailing between Ireland and Britain, Irish and British citizens can use a valid form of photo ID like a drivers licence. On a lot of domestic ferry sailings a valid form of photo ID will be enough or no ID will be required at all to check-in. You can find information on what form of ID you'll need to travel in your confirmation email.