The Gaels named Dùn Èideann from the original area Din Eydin which is believed to be an old Brythonic (old Welsh) name for the area around the rock, predating both the Gaels, then the Angles. Dùn of course means fortress, so Dùn Èideann could be "fortress of Èideann (Eydin)". Scholars are still unsure but believe Eydin refers to the area that was considered the ancient Britons' northern boundary.
In geographical terms, if you were to draw a rough imaginary line from the River Clyde across to the River Forth, everything to the south of that was ancient Briton who spoke an ancient form of Welsh. And everything to the north was ancient Pictland who spoke what scholars call Pictish...... they have no conclusive evidence but believe Pictish may have been similar to the Brythonic language to the south. All of this prior to the Gaels coming from the West around 500AD, and the Angles crossing Hadrian's Wall from the SouthEast around 700AD ........the Romans had departed from Brittania (modern day England and Wales) around 400AD.
Sorry, didnt mean that to be so long.