Virgin Atlantic had concluded an SPA (Special Prorate Agreement) with Aer Lingus/IAG, which negotiation had commenced prior to approval of the takeover.
This was also examined by the Commission, and approved at the same time as the merger.
IAG/Aer Lingus is also bound to offer similar PSAs to any other airlines that want them, as far as I can gather, but this is the only PSA mentioned by the Commission.
(While recently searching for biz class fares from DUB to ORD, EI/VS was the cheapest combination)