In your opinion...
which Irish dictionary is the best? Why?
my irish teacher always uses focloir.ie because it was made by foras na Gaeilge (an official group to preserve and promote the Irish language). It also gives sample pronunciation by native speakers in each of the three dialects (Connaught, Munster and Ulster). Focal.ie is another good one made by DCU (one of the top Irish Universities). But personally I find the first one better. Hope that helps, and good luck with learning Irish, or as we say as Gaeilge ( in Irish), Adh mór ort (:
Focal.ie is... technical. It's good for Civil Service people who need to know how to do something for a document, but not great at mimicking natural speech.
… which begs the question, best for what? I agree with Fingolfin1346’s summary, but would also add Dinneen’s Foclóir Gaeḋilge agus Béarla for assistance with older spellings (e.g. prátaiḋe for prátaí ) and for vocabulary no longer commonly used in daily life (e.g. preaḋain — the bones taken out of pork when it is to be cured as bacon).
I find that foclóir.ie (the New English-Irish Dictionary) and http://breis.focloir.ie/en (Ó Dónaill's Foclóir Gaeilge Béarla and de Bhaldraithe's English Irish-Dictionary) have different strengths but are both essential.
Foclóir.ie is more up to date so you'll find the word for blogger (blagálaí) and is good for when you want to make a fine distinction in sense e.g. do you mean 'for' as in 'this cake is for you' or 'for' as in 'there are speed bumps for 2km'.
Breis.focloir.ie/en is helpful if you're looking for Irish to English translations, has very good grammar charts for verbs, adjectives etc and is also helpful for finding idioms and alternative words.
If you're a learner looking for a printed dictionary, I would go with the Foclóir Póca published by An Gúm. It is a pocket-sized but comprehensive two-way dictionary with plenty of examples of usage. If you prefer larger type, the same material is also available as the A5-sized Foclóir Scoile.