Thank you soy and ROKNA! All was well in the end.
We were able to check-in fine at the counter in person. As advised we were seated together - window seat no less. I find the flight attendants polite and attentive. They went out of their way to make sure everyone was comfortable. They served ice-cream on the plane too! My husband was impressed. The flight was maybe a bit more than half full.
Interestingly they had told us they could check out bag through to LAX at SCQ counter. When we got to the gate to board, however, the machine made a sound when we scanned our boarding pass and the gate agent said "We had been waiting for you!" They took our LAX bag tags away and gave us new bag tags for DUB that were right next to their screen.
When our flight arrived at DUB we tried to check-in our bags then but they said we could only get our bags checked-in no more than 3 hours before our flight. Fair enough. We came back the next 4 hours before the flight and they were able to take them at the counter.
The US pre-clearance facility was very fast. There was no line when we got there. There were a few flights all leaving at the same time for US though - LAX, Hartford, Denver and Nashville. I could see how the lines could get long if we didn't arrive early.
We had a lovely time in Dublin and would absolutely fly with Aer Lingus again.