Lughaidh wrote:
Online : faclair.com
paper: Colin Mark's Gaelic-English Dictionary
Angus Watson's The Essential English-Gaelic Dictionary
they are the best ones, in my opinion.
Angus Watson has also put out
The Essential Gaelic-English Dictionary as a companion volume to the English Gaelic one which Lughaidh mentioned. Both volumes are comparatively recent (first published in 2000 and 2001) and still in print, I believe (I got them both through Amazon), and are very good, because they reflect current Gaelic usage and spelling (while still mentioning some alternate, usually older forms for words), and including cross-references to other word choices for a given concept. They also give a number of examples of how to use words in phrases.
For a very comprehensive dictionary which gives more extensive examples and usages, and includes more older words (or older spellings) which you may come across in writings and songs, there's also Dwelly's combined
Faclair Gaidhlig gu Bearla/Illustrated Gaelic to English Dictionary, published originally in 1901, although I have a 1994 reprint from Gairm Publications in Glasgow.
If you like etymology, MacBain's
Etymological Dictionary of Scottish-Gaelic is great, both for Gaelic and Irish, because it explains the etymology going back to Old Irish, and also mentions cognates in other Indo-European languages, which can be fascinating. The original was published in the late 19th or early 20th century, but I have a reprint published by Hippocrene Books in 1998.
For future use, I'm going to add a link to this thread in the Gaelic resources thread which is "stickied" at the beginning of the forum.