What's the best / easiest way to get from the Shannon Airport to Ennis?
By bicycle or taxi.
Bus! We had a Rick Steves tour last October that started in Ennis. Just ask the information booth at the airport where the bus picks up, and where the machine is to buy tickets. Very, very easy. Took about 30 min to get to Ennis once the bus picked us up, then a short 10 min walk to Grey Gables B&B from the bus station.
If you have Rick's Ireland 2020 guidebook (the bigger one), see info about bus #51 on pages. 324, 327, and 331.