Icelandair, Aer Lingus, and Alaska Airlines have round-trip flights from Portland (PDX) to Dublin (DUB) for $321-$375 with a connection. Flights October to December. 7-day minimum stay required. No Christmas dates available.
Some small OTAs on Skyscanner have this for cheaper than the airline. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.
Cheapest Skyscanner dates are Oct 24 – Nov 06 / Dec 09 – Dec 16.
A Carry on bag (20-lbs @ 21.6 x 15.7 x 7.8 inches) and a personal item (15.7 x 11.8 x 5.9 inches) is allowed for free on Icelandair. You can avoid seat selection fees by accepting the randomly assigned seat at the gate.
Icelandair offers free stopovers, which good if you want spend a few nights in Reykjavik. More details are on their website (link).