Based on what I've heard regarding a family member's experience, it seems Aer Lingus is not correctly encoding PreCheck into the boarding pass barcode.
This means PreCheck will *not* work with the older system of scanning your boarding pass while the agent visually checks your ID. It is working with the newer "CAT" system where you or the agent inserts your ID into the machine and it automatically verifies your flight details and Precheck eligibility. This seems to match the experiences of others in this thread.
TSA agents at ORD seem to be at least aware of this and are directing EI PreCheck pax to a line that has a CAT unit.