Before the hold expired at midnight, I called United.
I did receive an email today saying.
We have requested confirmation of your flights from Aer Lingus. This process may take up to 24 hours. We will notify you via email upon confirmation.
This reservation is not ticketed. Pending confirmation from Aer Lingus.
I asked if they could look into my reservation to see what the problem was before I transferred my miles to complete the booking. After a short wait, the representative confirmed that the space isn't actually there. I thanked him for saving me from wasting my time in trying to complete the booking.
The flight wasn't until next September and I already have a backup on BA First with AA miles. While I am looking forward to that flight, I'd be glad to rebook without the fees.