I don't think it would ever be worth it with regards to earning avios. For a cheap business class return you are talking about

LHR-JFK as a GCH or higher in J gets ~19000 avios
DUB-JFK in J gets ~8000

However, as you say if the EI price is good then go for it. At least you get 8000 avios, although obviously a lot less than with BA. EI regional have just started flying from LBA so I need to keep an eye on EI sales as I could see myself trying it out if the price is good. Also I think EI are adding more US destinations as well.

In terms of redeeming, you may struggle to get reward availability on NQY-DUB but DUB-JFK may be better. Also you may want to keep the flights separate as I would have thought you would pay APD (£142 from April next year) if you book it as a trhough ticket from NQY-JFK.