The island is called Runö in Swedish.
Prior to 1944 the island was for centuries populated by ethnic Swedes and traditional Swedish law was used. Ruhnu has currently less than 100, mostly ethnic Estonian permanent inhabitants who like to keep to themselves.
A ferry runs from Munalaid in the mainland to Ruhnu. The trip takes approx. 4,5 hours. Bookings can be made via phone and internet .
You can reach pretty much everything by walking. If arriving by airplane, there is a free shuttle bus to the village. Tell the bus driver the name of the place of family where you are staying, he will take you there. The same for getting to airport - ask your host to call the bus, it will pick you up.
The island is popular with hikers and swimmers, the best beach for swimming is Limo beach, with its "singing" sands.
The official Estonian tourism website provides an extensive list of options for accommodation in Ruhnu.