Easiest way would be to take the bus from Dubrovnik to Split and then jump on the next ferry or catamaran out to Brac. Not sure what time your flight gets in but the bus ride would take about five hours from Dubrovnik. The direct catamaran out to Bol would leave at 1600 and get to Bol at 1710. If you missed that you would have to ferry out to Supetar and then find transport to Bol. The ferries run all day so you will have no problem getting on one of them.
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