From my limited understanding (and I speak neither of the twa), they are in the same language family, but there are enough differences in usage, vocabulary and pronunciation that the two are quite distinct - like Spanish and Italian are in the same family with each other, but they are different enough to be mutually unintelligble.

I have read that, given enough time, patience and a broad enough vocabulary, speakers of each form of Gaelic could garner a limited understanding of basic phrases, but not really enough to hold an in-depth conversation.