Cheapest may be 25k Avios RT via BOS>DUB on Aer Lingus (booked using BA Avios). Of course you will have to find other means to/from your gateway city (BOS). You can also do this ORD>DUB RT for 40k Avios.
Some additional info here:
http://boardingarea.com/onemileatati...on-aer-lingus/
Some more options are mentioned here:
http://boardingarea.com/onemileatati...region-europe/
Though no personal experience with this, I have read that intra-Europe flights via Avios are usually a good bargain too (most are 4,500 Avios each way I believe).
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