Icelandair has round-trip flights from Denver (DEN) to Dublin (DUB) for $480-$622 with a connection. Flights vary depending on destination, see the example date list for more details. Most flights are between September and April.
Destinations are ordered by price. Dublin is the cheapest, Berlin is the most expensive.
Note: Some small OTAs on Skyscanner have this for about -$25 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.