Sorry about this. It only occurs if the property is vacant but electricity is being supplied. It serves as a reminder in cases when there are new occupants who forget to register when they move in. It is a standard letter for any company that is supplying energy to the property. There is a certain point where registration would have to be with them if they have been supplying the property with energy for some time (but the customer could just register for that period of time, and then switch).
If you wish to ensure that your registration with the other company is complete, either message us those details to see whether the property's energy is now no longer being supplied by Electric Ireland, or just check with your own company that your switch is fully processed.