If you do a ticket that includes DUB or SNN on it in the fall/winter it will lower the fare a lot. So, if you did say JFK-DUB/CDG-JFK it would be cheaper than JFK-CDG-JFK, because of the cheap fares to DUB in the off season. I just booked a LAX-MAD/DUB-LAX and it priced out about $300 cheaper than LAX-MAD-LAX. I checked some other cities too (BRU, LHR, FRA, etc) and it was the same thing.