You'd be looking at a few mins walk from Clyde Road (really depends what end of it you're on - it's quite a long road) to Ballsbridge, walk around the American Embassy, and wait for either 45,7,8,84 or anything that goes by that stop which will take you to the quays. Then another 5mins to Busaras.
Basicly, using buses, you may or may not make it on time, really all depends on how long you end up waiting for one. In a taxi, it's a 5-10min trip
30 minutes isn't leaving you much space to rely on a bus.
You are likely to get a better bus service (10, 46A/B to O'Connell Bridge) from Morehampton Square on Morehampton Road (continuation of Upper Leeson Street to Donnybrook Road) than Ballsbridge (7, 45, 46 to O'Connell Bridge - these tend to arrive in fleets at half hour intervals).
You **should** be able to get a taxi on the street. Alternatively there is a taxi rank at Jury's Hotel Ballsbridge. Taxi should be about €6 (more if you phone for one).
