Professor Tamara Hervey:

That would work. Legislation is not currently in place but there is a contractual arrangement underpinned by EU law, through the directive. The directive will be removed but Ireland can then interact with the UK as a third country. While it cannot use the administrative structures of EU law to support the movement, I believe such structures are currently being used to do so. It is a movement under the directive, not under the social security co-ordination provisions. It is more difficult, therefore, to be sure that EU law is being used administratively. I have tried to determine whether it is, albeit without much success. It is clear in the withdrawal agreement that the UK will not be able to use the mechanisms used for social security co-ordination. I may be speaking out of turn but it is a ridiculous waste of resources, although it is just one corner of the ridiculous waste of resources that is Brexit. Legally speaking, the EU cannot allow the cherry-picking of convenient arrangements, and I do not believe that it will.