An update on my last post -

I called the number United showed at checkout for assistance. That's apparently web support and while they can technically book tickets, they were having no luck. I called later directly to reservations and the agent was helpful, tried to build the itinerary and book it and ran into issues. She "checked with her resources" and said it wasn't available. I was uber polite and asked if a supervisor could possibly help. I was transferred and the supervisor seemed to know why it wasn't working and set up the reservation and said it would be confirmed within 24 hours once they heard back from Aer Lingus on availability. Two hours elapsed, but victory was in sight, I thought.

Jump ahead 25 hours and I called back. I had not seen it ticketed yet and the confirmation number provided to be by the agent wasn't on my UA profile, but two others were, one of which was shown as cancelled. The other one was right but was in all economy (it was supposed to be a mixed cabin award).

Same drill as before. Agent was helpful, he tried again, checked with "his resources" and said it wasn't available. I asked if I could speak with a supervisor because I didn't want to take any more of his time. He first spoke with one, supposedly, then I asked if I could and I was transferred. Again, the supervisor seemed to know what was going on, rebuilt it, and tried again, this time for a different date. I held while they supposedly reached out to Aer Lingus who said the availability wasn't there. 1.5 hours elapsed time and I threw in the towel.

TL;DR - UA awards on Aer Lingus from YYZ to DUB seem to not be "real" right now. If you want to try, find the availability online via UA's website, try booking, then call reservations (not web support) and get to a supervisor to help. Godspeed.

Postscript - I found a better routing, all on UA, for even fewer points. Completely different dates, but it still works for us.

In the past, I could book BOS to SNN or DUB on a non-stop partner award with Aer Lingus fairly easily. Now, all flights that show for me require at least one stop. I wanted something towards the end of the booking calendar but checked other months too. I don't have status, just a Mileage plus credit card. Am I missing some trick or is that route impossible now? Thanks!

UA partners decide when (and how much) inventory to release for booking -- it's possible/likely that EI hasn't yet released any inventory/the inventory they've released has already been booked. EI may or may not release additional inventory closer in (anecdotally when I've looked at changing Europe-pointed award tickets within a few days of travel, historically, EI has had some of the better points fares when those weren't available at original booking.