OC Richard is correct. It would be wrong to say there are "6 Gaelic languages." Gaelic refers to the Q-Celtic tongues. So calling all the Celtic languages Gaelic would be a bit like calling all Romance languages Spanish.

As to the difference between Irish and Scottish Gaelic, I asked the same question of a linguist once and was told that prior to 1700 the differences were minimal (effectively different dialects of the same language) but post-1700 the two began their own trajectories and are now considered different, though closely related, languages. So, rather like Spanish and Portuguese, native speakers of each could converse and understand a fair amount of what the other was trying to say.