Aer Lingus publishes sizes for this space that disagree: 40 × 30 × 20 cm (baggage information); 15.5 × 11.5 × 8 in (baggage information). These candidates are within every one of them, using published exterior dimensions and no compression. Bags within only some of them are not listed.
Some physical allowances remain unresolved. Confirm the applicable allowance with the airline. This guide does not assume missing trip details.
These meet the recorded limits with 1.5 cm or less clearance on at least one axis. They are deliberately excluded from the strict list; remeasure the fully packed bag before relying on one.