Complex sentence: When the Porsche involvement ended at the end of the Group B days, Rothmans association transferred to Subaru who was run by David Richards' Prodrive team who ironically ran the Porsche rallying team and was sponsored by them when he was co-driving the Ford Escort RS1800.
Split into simpler sentences: When the Porsche involvement ended at the end of the Group B days, Rothmans transferred its association to Subaru, run by David Richards' Prodrive team. Coincidentally, Richards had run the Porsche rallying team and was sponsored by Rothmans when he was co-driving the Ford Escort RS1800.