Greetings from Ireland
Andy Steves and I took a group out to Inisheer last year and we loved it. We've both been several times.
Its very small and there are a few nice walks.
Ive never overnighted there, so cant comment on that aspect of it, I'd imagine and hope its fun. The pub we visited was a good spot.
However, in terms of archeology and sheer glorious scenery, you really cant miss Inis Mor, in my humble but expert opinion.
And choppy seas wouldn't be any more likely in April than at any other time. or the ferries not to operate, it needs to be almost Tsunami-like conditions.
Those ferries are the daily commercial lifelines of the Islands. They tend to operate even in pretty choppy conditions.
Enjoy the Islands!
Slainte
Stephen McPhilemy
Rick Steves Ireland Tour-Guide
Dingle, Derry & Dublin