you would be very unlucky to be stopped if you are in a car - I did the trip last week in a car and based on that visit we would only have been pulled over exiting the port at holyhead but they usually only stop vans or cars loaded up with "stuff". If I was in the same situation I would take a bank statement to show I lived in Ireland as ID is required to open an account. Saying that, in 12 years of travelling on that ferry I have never been asked for ID only sometimes a random "what nationality are you?"