OK, on our 2 trips, both times checked out of Galway the morning we headed to the Aran Islands. Each time we then stayed 1 night, and also had torrential rain, but still got time outside. You could potentially squeeze in a daytrip visit, but we were fortunate to do an overnight each time. You could skip the Aran Island visit, but it’s been an essential part of each trip for us.
1) June 2012, left our B&B in Salthill (suburb of Galway), and rode our bikes in a sudden rainstorm, west to the ferry docks at Rossaveal. The road was narrow, with lots of traffic splashing us. We didn’t go too far over to the road’s shoulder, because of the brick walls just off the edge. We took the next available ferry, bringing our rental bikes aboard. It couldn’t take cars; rental car tourists would have to park and board the boat on foot. We sailed south for Inishmore, known in Gaelic as Inis Mor, the biggest of the 3 islands, and the most visited. The prime sight is Dun Aengus, an ancient fortress at the edge of the cliff overlooking the sea. The cliffs erode, and part of the fortress has tumbled way, way down into the ocean. You can lie on your stomach on the flat rock and carefully stick your head over the edge, peeking down at the crashing waves. It requires a hike up a steep trail to reach the fortress ruins. A van tour will drive you there, plus to other sights, showing you the harsh conditions that locals have endured to live there for centuries, before tourism brought visitors. The day we arrived, with reservation for a night at a thatched cottage B&B (which no longer operates), it was reported that, even though it wasn’t raining or super windy on the island, the seas were so rough that ferries weren’t sailing back to the mainland. I’m glad we had a place to spend the night. We were able to book a ferry for the next morning, subject to calmer waters, and after a rainstorm first thing that morning cleared, the ferry headed east for the mainland. It stopped for an hour on Inisheer (aka Inis Oir), the tiniest of the islands. Some people grabbed a quick lunch. I unloaded my bike from the small ferry and pedaled out to see an old shipwreck that’s been washed ashore. The wreck appears in the opening credits of Father Ted, a TV comedy that was set on a fictional Irish island, and filmed in Irish locations. We finished docking on the mainland close to Doolin and Lisdoonvarna, not where we’d started. If we’d had a rental car, we’d have needed to sail back to Rossaveal on a return ferry. As we approached the shore, we got views of the Cliffs of Moher in the near distance.
2) For Sept 2019, we’d just picked up our rental car (no bicycles this trip), and drove partway towards Rossaveal. Nice to not have rain, but the brick walls by the edge of the narrow road were still there. We pulled over at the airport for the Aer Arran Islands airline. They fly very small but very capable planes to Inishmore. I think they’ve gone to all 3 islands in the past, but I think Inishmore was their only destination last time I checked. Flying is more expensive than a ferry, but there’s a major advantage. The second trip, the weather was nice during our arrival day, but it turned bad that night, and we spent our second day just sitting inside our B&B, watching the rain blow sideways, ripping the pretty flowers in their boxes to shreds. The storm cleared in time for our scheduled flight back to our car the second afternoon, and it was a fast, safe flight - 16 minutes. Ferry passengers were stuck, though. Even with clear skies, the seas were too rough for ferries to cross. Under the circumstances, I’d say fly, and see just the biggest, Inishmore. Spend the night if you can, or do just a daytrip, flying back to the mainland before nighttime. But if you fly back, you’d wind up west of Galway, and would still need to drive way around towards the Cliffs. Maybe get a ride to and from Galway and the little airport, then get a rental car afterwards.