I went there for two weeks last year and tbh mountain biking would be pretty shite as there aren’t any really big hills and the ones that there are don’t have clear path’s over them (not enough people!) and there are a lot of peat bogs, so you would be pushing a lot. Take a look at google maps + street view.
Its good for walking or road biking, and the beaches are amazing. Swimming is an eye opener as the water is a bit cold, also peak temperature is a bit lower, as when I went last June it peaked at 14C on the hottest sunniest day, and was 7 or 8 when the sun went in, and the wind is something to behold.
You would be very very lucky for bike hire of anything decent!
You also need to bear in mind there are very few people on the islands, and everything shuts on Sundays, and I mean everything!
Its a very special place though as I really liked it, but you need to be happy with you own company and not mind a bit of weather; I wouldn’t fancy it in the winter though.