It is definitely worth a visit - check out my photos from Ilha Grande here
A good site for instructions (although a bit biased sometimes - probably gets commission)
Find a bus that goes to Angra dos Reis there should be a few companies just ask around.
To get to Abraão on the same day you have to arrive to catch one of the buses.
Listed here as leaving only twice each day.
PS. I would recommend going with friend or making some there and then going on a diving tour on a small boat, it is a lot of fun almost 10 years ago
A bus line which has several departures to Angra dos Reis is Costa Verde. It has the below services and costs 35r
05:00h - 06:00h - 06:40h - 07:00h- 08:30h - 09:00h 10:00h - 10:20h - 11:00h -11:40h - 12:20h - 13:00h - 14:00h - 15:00 - 15:40h - 16:00h - 17:00h - 18:00h - 19:00h - 19:30h - 21:00h
From Angra dos Reis there is generally one ferry a day which leaves at 15:30 on weekdays and 13:30 on the weekends so make sure you get there in plenty of time.
I would say spend a minimum of 3 or 4 days here but beware - it is very easy to get stuck as it is absolute paradise!
Hope this helps, almost 10 years ago