Aaahh...I knew this sounded familiar...you were looking for seats a week or two ago, right? (Probably a good move considering to/from Europe in the summer books quickly.)

If it were me, I'd book DEN-BOS (UA, 25k MP), BOS-DUB (EI, 25k Avios), and DUB-BOS (EI, 25k Avios) now. Keep watching UA over time. When WN opens up, book BOS-DEN on WN, knowing you can always cancel it if UA opens up with a better flight. The lack of interlined bags on the way home probably wouldn't bother me a whole lot.

A 7-hour layover isn't the worst thing in the world: if all runs ontime, you go hang out in Boston for a few hours. If there are IRROPS out of Denver, unlikely on a June morning, you can relax knowing you aren't on a tight connection.