I posted a similar question on this subject in October but now (of course) have a new twist. For a variety of reasons, I may end up purchasing two separate tickets to fly from Portland to Dublin (PDX-AMS on Northwest Airlines, then AMS-DUB on Aer Lingus). Question: since I won't be able to check in for the AMS-DUB flight in Portland, how much time should I allow to check-in for my transfer flight at Schipol? I have options for either a 2 hour or 3.5 hour layover. Also, any updated info on carry-on limits on Aer Lingus - restrictions were eased in UK airports, but can't tell about Ireland. Thanks for your advice!
IF you have checked baggage that can't be automatically transfered to your AMS-DUB flight allow 3.5 hours. Otherwise 2 hours should be plenty of time. Near the gate for your AMS-DUB flight, there should be an Aer Lingus transfer desk; this is where you can collect your boarding pass.
For peace of mind, I would go for the longer layover. What's the on-time record of your flight from Portland? A couple of years ago we booked separate flights for our return to the US, and I got pretty nervous when the first flight [Vienna to Heathrow] was delayed about 45 minutes. We did make our return flight but it was not a good end to an otherwise great trip. If you end up with extra time in Schiphol, eat some breakfast or go visit the airport Rijksmuseum; it will probably feel good to move around after such a long flight.
Have fun and good luck!