Dwelly's was the standard decades ago. IIRC there is an older dictionary posted on Gutenberg.org, but it will be quite different from modern spoken Gaelic. Honestly, I don't know what the most recent dictionary is. Google is pretty good for individual words. More importantly what are you using as a primer? I recommend Roderick Thomson's Teach Yourself Gaelic. But you will need to hear it. There are Youtube videos of introductory Gaelic.

Irish and Scottish Gaelic have such a variety of dialects that there is no simple answer to that. Aran Islanders and Lewis Islanders are speaking different languages. But Islay Islanders and Donegal speakers are fairly close. Donegal Irish uses similar negatives to Scottish Gaelic, differing from standard taught Irish.