Absolutely, definitely, for-sure fly rather than taking the ferry to Arann Islands. It's cheap, spectacular, super-fun, and you are in the air for less than 10 minutes (seriously, it's the shortest airline flight you will ever take). Plane is very small though (this is a feature, not a bug). So for those prone to motion sickness, take it as seriously as you would a boat -- but the difference here is that you would only be susceptible to discomfort for under 10 minutes, versus a much, much longer time on the ferry (what, an hour? I only know it's a slow boat).
If you are hoping to go to Skellig Michael (sounds like you might be), it's not realistic to say "only go on a nice, calm day with no wind." You go when they tell you that you can go. The conditions are usually (almost always) rough. If you wait for a calm day with no wind you'll be waiting forever. You will need to deal with the choppy conditions, or you won't be going, period.
Good news: there is one (and AFAIK, only one) highly effective way to beat motion sickness: the scopolamine patch.
I speak from many years of intense, intimate experience -- not my own need (iron stomach here, I actually enjoy that "roller coaster" feeling in the gut), but my dear spouse, who has literally gotten motion sickness in the 3-block drive from home to the nearest grocery store (she says it's the way I drive...). I have never met another person who is more prone to motion sickness than her. Five minutes in a boat (even large ones), small airplanes, cars, buses, trains (yes, trains!), even just sitting in a theater and watching a movie that has too much fast camera movements, wearing a VR headset -- never mind riding on a real roller coaster or even very tame fun-park rides -- she quickly goes green and the rest of the world soon gets some color added. She has struggled with this all her life.
To add to the challenge, I enjoy lots of activities that involve being in conditions that would quickly push her motion sickness buttons. We are scuba divers, and diving requires getting out on very small boats, often bobbing in one place for long periods, sometimes going in very small rowboats or tiny "Zodiac"-type inflatables. We tried every method under the sun to suppress and combat her tendency to "feed the fish." Every suggested "cure" quickly (usually immediately) failed, all seemed to have exactly zero effect. Nothing worked. At all.
Then she tried the Scope patch. THAT worked. Like magic. You do need a prescription to get it (at least in the USA), so check with your father's doc. It's like a miracle, but it does come with some (generally) minor side-effects: dry mouth, also can make you slightly drowsy; be careful not to touch the patch then touch your/someone's eye, if you do the medicine can cause blurry vision. It's serious stuff, so before relying on it "in the wild" do a test run at home under controlled conditions. Also -- IMPORTANT -- it takes hours to become effective, so you must not wait until it's too late to slap the patch on, you MUST take it hours before you will need it. My wife puts the patch on before going to bed the night before she's going on a boat. The patch works for about 2-3 days, giving great relief. It has literally changed her life.
If your dad wants to do the boat out to Skellig Micheal (which is wonderful and memorable, don't miss it if you can make it work), you will need some serious solution for someone who suffers from seasickness. "The patch" worked perfectly for my wife, and she had a wonderful time on Skellig Michael.
Hope that helps. Good luck.