Should I try to buy ferry/air tickets ahead of time or wait and see what the weather holds the day I want to go/return?
The chances of the ferry from Rossaveel to Inishmore being cancelled because of the weather are quite low. It's a much larger ferry than those running from Doolin and, therefore, can better handle the choppy waters of Galway Bay. It also has indoor seating. Have you reviewed all the details on its website?
I haven’t yet but am leaning towards the ferry. Right now I’m booking travel links. Air and accommodations and car where I need it are booked.
You don't need to book tickets in advance. The ferry hardly ever fills up so you can check the weather forecast a day or so prior to your trip before committing to a particular day.
As was mentioned the ferries are sizeable craft (about 100 ft in length) but they will suspend service in the event of high winds or rough sea states - conditions when you wouldn't want to be aboard anyway.
If a rough crossing is a concern on a day when the weather looks otherwise favorable an alternative is to fly, which several people here have suggested is a quicker, more comfortable and more scenic way to go. Doesn't look all that more expensive than the ferry either. We've bookmarked it ourselves for future reference: https://aerarannislands.ie/
The ferry has a shuttle bus which leaves from downtown Galway. I found this very helpful as we did not rent a car.