The Pax traveling to Dublin if going via EDI will have to clear U.K. Border Controls as first point on entry to the CTA. Same applies if flight arrives in DUB all Pax will be seen by the Garda. It's a very complicated route hence why long delays in it being announced. The mixing of Pax will mean cases of passport swaps and inadequately documented arrivals at EDI and DUB. China asylum claims is a major immigration problem for U.K. & Eire. CTA rules add extra Visa & Custom problems.
Also CAA rules require rescreening of all international passengers to U.K. Standards.