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The Irish forward broke the deadlock in the dying seconds of the first half when he headed home from six yards following a good cross by Landon Donovan.
Mike Magee doubled the hosts' advantage on 64 minutes with a low right-footed shot from the centre of the penalty area.
Keane then completed the scoring three minutes later with a close-range tap-in following good work by Christian Wilhelmsson.
Galaxy look likely to face Houston Dynamo in the MLS final, after the Texans beat DC United in the first leg of their Eastern Conference showdown earlier today.