Ryanair cabin crew are not working on Aer Lingus aircraft. They'd have to go thru full training for the aircraft which takes time and that's only from a safety point of view. It would be the same if Aer Lingus crew were to go working for Ryanair. That's a poor rumour that needs to be knocked on the head now.
As regards the other question about training for an aircraft. The short answer is that all aircraft are different and cabin crew need to train up on each individual type as the location and use of certain safety and emergency equipment and door types are different. A B737 is so different to a A320 as is a B777 to a A330. Perhaps to the average Joe Public, on first glance, there is no difference but there most definitely is.