The answer
Take Bus Éireann 343 from the stop outside the terminal to Limerick (via Shannon town and Bunratty, into Colbert Station) — the Mid-West's first 24/7 bus route, running every 20 minutes at peak and every 30 off-peak, with about €6–7 the indicative adult single.
Shannon sits on the main Galway–Limerick–Cork road, not on a branch, so scheduled buses pass the airport around the clock; the NTA's Connecting Ireland upgrades made the 343 an every-20-minutes shuttle to Limerick that keeps running after the last flight.
Tight budget? A Leap card gives the cheapest fares across the whole TFI network — top up on arrival.
In a hurry? Taxis wait for every arrival and make Limerick in ~25–30 minutes on the M18 — but the airport publishes no fare table, so expect a negotiated ≈€40–60 (range, unverified).
⚠️ The traps (walk past these)
There is no train at Shannon
Nearest rail stations are Limerick Colbert and Sixmilebridge — both road-only. Bus Éireann's route 316 exists precisely because Sixmilebridge is the closest station to the airport. Any guide that shows a 'Shannon rail link' is recycling Dublin's map.
Do this: plan bus-or-cab only; change to rail at Limerick Colbert or Sixmilebridge
The exact '€6.80' you saw is unverified
Multiple aggregator sites still quote €6.80 (others €6.08 or €9.50) for the 343, but Bus Éireann publishes no static fare for it — price follows the TFI Leap zonal system and payment method. Budget €6–7, tap your Leap, and ask the driver rather than trusting a blog decimal.
Do this: treat every exact single fare you find online for the 343 as indicative, not binding
The 343 is a meander, not a motorway dash
It winds through Shannon town, Bunratty and industrial estates: the official timetable books ~38 minutes from Limerick to the airport, and third-party trackers say 45–55 in traffic. And for departures, Bus Éireann's own Shannon page tells passengers to arrive at the airport three hours before the flight.
Do this: add the buffer — bus time plus check-in time — on any tight connection
'24/7' is not 'every 20 minutes at 3am'
The late-night service is real but thin: hourly between midnight and 4am and hourly Sundays (20-minute peak frequency is weekday business hours). Miss the 02:48 and the next bus is 03:48.
Do this: check the exact off-peak timetable before counting on a 3am bus