Yes, they have a common ancestor but there are slight differences.
I don't know alot about Gaelic, but the place i live is in Gaelic and we have a hell of a time giving out our address because of it!
We have to spell every word letter by letter.
From my limited grasp, Scots Gaelic is a bit more like 'French' than its Irish counterpart; it doesn't pronounce hard sounds. For instance, in Irish Gaelic the city of Dublin is spoken to sound as it is - in Scots gaelic it would be pronounced 'Doo-lin'. We would drop the hard 'b' sound. Like French, Scots Gaelic likes harmony and flow. But both mean the same thing in English (Black-pool). Somebody correct me if i'm wrong!
Where i live in Scotland, in ancient times they spoke Pictish, not Gaelic. It never made it to written form as far as the experts know and it died out. Some academics think it would have been closer to modern-day Welsh, though nobody can be sure. All thats left of the Picts are ancient standing stones they carved with symbols and decoration which litter the North-East of Scotland. Unlike the Celts who favoured ornate swirl designs, the Picts enjoyed depicting animals. I remember i seen a pretty one once where they'd carved a deer with its calf sheltering between its legs. Sweet.
Sorry, am delving off topic here...