We are flying BA from Dublin to London and then on to Seattle. Will we still go through customs at Dublin or when we got off the plane in Seattle? Trying to figure timing, last time I tried to check in for A BA flight home no staff there to check in until 2 hours before the flight but that was in France.
No Preclearance for you. You'll go through immigration and customs in Seattle.
Fill in your flight information on this website and it will give you step by step instructions on what to do:
http://www.heathrow.com/flight-connections
But as noted, everything will be done in Seattle. Only direct flights from Ireland get pre-clearance.