Scots Gaelic (Gaidhlig) is eerily similar to Irish Gaelic (Gaeilge). There are some differences in grammar, spelling, stress marks, etc.
I'm not sure if Irish can understand Scottish. Up in the Ulster Province of Ireland, I think they can understand Gaidhlig. (Ask Conor).
I have both a Gaelic and Irish grammar book and the content tells me that the two languages are very similar. They both come from the Celtic language family. This includes Welsh, Manx Gaelic, Scots Gaelic, and Irish.
I'm not sure which language is oldest, but I think it's probably either Irish or Scots. There are some people on the forum that can probably answer yar question better than myself.
Ádh mór ort! Slán go fóill.

Ní bheidh Éire shaor ar síocháin choíche, agus gan an ceart, ní féidir an tsíocháin a bheith ann.
Tomás Ó Broin
Learning Irish since October 2003