Niall O'Donaill

A nice, thick, pretty much complete dictionary, presenting Irish words and glossing to English. Note that it doesn't go the other direction--you'll need to look in another source to translate English to Irish. One nice feature of this book is that many of the tricky genitive and plural forms of nouns, which may not entirely resemble the singular/nominative, are cross reference back to the basic form. That makes it a good resource for novices who have not yet learned irregular declensions

If you're choosing an Irish dictionary, you may find this
comparison of popular Irish dictionaries, with sample definitions helpful.

A modern Irish-English dictionary (Gaeilge = Irish Gaelic, Béarla = English). Ó Dónaill uses the Latin alphabet rather than the traditional uncial script, which was pretty much abandoned after the mid-20th century. A hefty and very useful dictionary.