I'm reading this volume to fill out the stories not in translations by Jeffrey Gantz, Elizabeth Grey (Second Battle of Moytura), Thomas Kinsella (Tain), John Koch and John Carey (Celtic Heroic Age), Ann Dooley (Tales of the Elders of Ireland). It has a good selection of stories about Finn mac Cumhaill and about the Irish kings. The translations are all by the greats of the past (e.g., Whitley Stokes, Kuno Meyer) and are classics in their own right.
This is an extremely useful book for anyone wishing to simply read English translations of Irish mythological texts. It contains a good cross section of stories from all four cycles: Mythological, Ulster, Fenian, and Historical/Kings. The language is a little dated, and better, newer translations of some of the material exist, but for all but the Ulster cycle that material is scattered through hard-t0-find publications which are time-consuming to track down.
This is a big collection, and contains more than enough material to get you started reading Irish mythology as it occurs in the great Medieval texts. I'm not aware of a more comprehensive collection.
Great because it is free of the typical interrupting academic analysis and gives the myths without interruption. Hard because the language is archaic and difficult to work through at times. A romanticized writing but still worth the read.
This has some very interesting post-Christian stories. If you read pre-Christian Celtic stories you will likely be interested in how the heroes of the Celts become the heroes of Christianity.