Just stumbled onto this. Most of my family on one side are from stornoway and speak the gaighlig as a first language and my girlfriend from ireland, who learnt some in school was able to pick up one or two words but didn't really understand much at all. I have heard that the gaighlig of Islay is similar to ireland. Also there has been a history of antagonism towards gaidhlig in scotland from the south and this led to a distancing of the language, ie disparagingly calling it erse. Rececently more and more academics tend to believe that gaighlig has been in scotland far earlier than the 5th century due to more compelling archeological evidence rather than reliance on the dalriada myth and conquest of the house of alpin myth, believing the dividing line in scotland was up the spine of scotland with p celtic on the east and south and q celtic to the west and north. In theory the west of scotland may have been populated at the same time as ireland with 'gaelic' speakers although it is likely both travelled through iberia due to genetic links.
Tapah Leibh