I use faclair.com aka Am Faclair Beag which is an online dictionary encompassing Faclair Gàidhlig gu Beurla by Edward Dwelly (an old, early 20th c., but one of most renowned dictionaries) and Faclair nan Gnàthasan-cainnte formerly hosted on the Akerbeltz website, and additionally adding more newer entries of its own with pronunciation hints and recording. The site is pretty rough and might be hard to navigate before you get used to it, so I recommend looking at its Help page.
Besides this I use The Gaelic-English Dictionary by Colin B.D. Mark – this one’s much more modern (2003) but commercial. You might be able to browse it a bit through linked Google books (the search inside feature sometimes lets you find relevant entries), but to use it regularly you’d need to obtain a copy (it’s available both as a physical book and an e-book – I believe several formats are sold: Kindle (mobi?), epub, pdf…). It has many usage examples besides simple entry translation and a short summary of Gaelic grammar too.
You can also browse all Gaelic dictionaries available online at multidict.net.
Besides those, Wiktionary might be helpful, and sometimes Irish dictionaries might come in handy (especially teanglann.ie is my go-to Irish resource, also Dinneen’s, also here aka Foclóir Uí Dhuinnín is good for older dialectal (Munster) Irish forms).
For other recommendations, see the The Best Gaelic Dictionary for You list at gaelic.co – it also recommends Am Faclair Beag and Colin Mark’s dictionary but also has a little warning about the latter. And recommends a dictionary I don’t have experience with: Angus Watson’s The Essential Gaelic-English / English-Gaelic Dictionary which might a be a good Colin Mark’s replacement.
It also highly discourages using MacLennan’s. With those quotes:
“I gave up on MacLennan’s very early on, do not advise it to anyone.”
“MacLennan’s, to quote a friend, ought to be pulped or banned or both.”
“I begin every year telling [my students] ‘Maclennan’s’ Dictionary ought to be burned, and further reprints banned – a mean con-trick played on learners trying to steward their limited finances thus brought to an end. But they probably just wonder what I’m wittering on about a printed book for.”
Very useful. If I made add my own experience...
Not only is this bad but it is old and bad. Modern reprints are profiting from the fact that it is out of copyright and essentially conning people by selling something that is free, not making it clear it is 95 years old, so it does not even attempt to represent modern Gaelic.
I have never heard or seen anything good about this from anyone with the knowledge to say if it is accurate. It often seems good to learners because it is simple and clear, but there are serious inaccuracies in it, some of which have been discussed on Duolingo. See https://forum.duolingo.com/comment/36956069. Its main disadvantage is that is has nothing to recommend it over the free options available on line to justify paying actual money for it.
An excellent dictionary with two big negatives. One is its enormous price. But if you can borrow a copy (preferably digital) from a library then great. The other is that it is Gaelic-English only. But as I would never use a printed dictionary these days if I had the choice then this is not an issue. With the digital version you can simply search for an English word, and it will not only find all the Gaelic words with the English word in the definition, but it will give you all of his copious examples that have that word in the translation. For example, if you look up the word tasty (or the word blasta) it will not only find you the definition of blasta but also four examples in very natural Gaelic of how to use the word in a real sentence. In fact if you read the whole book from cover to cover, you would probably end up with perfect Gaelic.