Irish Gaelic and Scots Gaelic are a bit like Italian and Spanish - fluency in one goes a long way to understanding the other. I am learning Scots Gaelic and can read some Irish with a bit of difficulty, but fluent speakers of Scots Gaelic can more or less understand most Irish - indeed Irish Gaelic is often broadcast on Scots Gaelic radio. The people from Islay however have a Gaelic that is almost a cross so have less trouble than the rest of Scotland. Donegal Irish is the closest to Scottish Gaelic. The written form of Gaelic is easier to understand than the spoken form due to being more standard.