Hi, I just called RTA on this today and was given the following timetable information
There is also an E101 bus service which leaves Ibn Battuta metro station to the bus station at Al Wahda Mall, Abu Dhabi
Departs Dubai 6am, 7.30am,9am,10am,4pm,5.30pm,7pm and 8pm
Departs Abu Dhabi 8am,9.30am,11am,8pm,9pm and 10pm
It departs from Al Gubaiba Bus Station (next to carrefour in Bur Dubai) and they leave as soon as they fill up with passengers. the cost is 20 dhs and they are very nice buses. They will drop people at several stops starting from Yas Island turn off to downtown Abu Dhabi Bus Station. You can also take a "taxi" van which costs more, but is faster.