Cruising between GB and Ireland post Brexit
Excerpted from an article by Norman Keane
•    There are no Customs or immigration formalities when crossing between the Republic of Ireland and Northern Ireland in either direction, nor between GB and Northern Ireland (except in respect of pets – see below).
•    When arriving in Great Britain direct from the Republic of Ireland, a call must be made to the National Yachtline on 0300 123 2012 and a copy of Customs Form C1331 Part 2 completed and posted to the UK Border Force office in Dover. Refer to Customs Notice No.8 on gov.uk. Flag Q must be flown until the National Yachtline has been contacted.
•    Arriving in the Republic of Ireland from GB, a call must be made to the nearest Customs office (details in the Irish Cruising Club Sailing Directions and at https://www.tariffnumber.com/offices/ie). Flag Q must be flown until this is done.
•    Carrying pets (officially includes dogs, cats and ferrets) on boats is now problematical. Dogs must have a specific tapeworm treatment one to five days before arrival, and pets may only be brought into Ireland at Larne, Belfast, Dublin, Rosslare and Cork (Ringaskiddy). The commercial port areas of these places are de facto prohibited to yachts. Note that RoI and NI have adopted exactly the same protocol. The two Departments of Agriculture have confirmed that it is now impracticable to bring a dog or a cat to the island of Ireland on a yacht. There are no such restrictions in GB, so although you can take your pet to Britain, it’s very difficult to bring him or her back.