As Brian said BA Avios and EI Avios are completely interchangeable and makes no difference what account they fall into. In terms of what credit card to use, go with the one that gives more Avios and/or associated benefits (or get both if you can and double up on rewards if you can hit the spend targets). In terms of flying to Ireland, yes using Avios on BA booking site or avios.com can be expensive with the fees but you definitely save money compared to cash ticket long haul. In general I believe it is cheaper to use EI compared to BA as their fees are lower. It gets quite complex IMO on the best way to use avios for EI redemptions, I think the best way is to use BA booking service and ring them on the phone to get the lowest fees for EI avios redemptions. Sitting on 400K avios I'd suggest getting business class seats as your avios give way more value that way but it is very difficult to nab 2 business class seats on EI award bookings.