As a different Plan B, there are 2 seats in F IAD-LHR (on the A380), for not much more than the double-WT+ cost.

You can hop down on US but have to take the domestic fairly early (LGA-DCA 1400-1520 I think) and then schlep from DCA to IAD (depart 1830), so all-in-all if possible I might fly down the night before or early morning and have a day in DC.

If you're heading back to Ireland you might want to call and ask about Aer Lingus availability, from NYC and BOS, as it doesn't show up online.

There is one WT+ available for normal redemption on the 22nd btw (1930).