Reddit mentions of Book of Conquests

From a previous response:

Irish Mythology (as opposed to more recent Irish folklore) is divided into four 'cycles'. Each cycle contains tales dealing with certain subjects or characters.

• The Mythological Cycle deals with the foundation myths of Ireland; the Tuatha De Danann, the Formorians, etc.
  
• The Ulster Cycle deals primarily with the deeds of Cú Chulainn, which are encapsulated also in The Táin - the 'Illiad' of Irish mythology. It also, however, contains tragedies such as Deirdre of the Sorrows.
  
• The Fenian Cycle is like the Ulster Cycle in that it deals with heroes and their deeds, but has a distinctly less epic feel - usually concerning distinct incidents in the lives of heroes such as Fionn Mac Cumhaill or Oisín. It also relates another favourite Irish tragedy, Diarmuid agus Gráinne
  
• Lastly is the Kingly Cycle, short fables that impart the qualities of great kings in the face of difficulty.
  
  Pretty much any and all of these tales are available in academic form online, but it makes it much more enjoyable to find a good prose translation by a good author. You can buy The Táin on paperback here, and Jim Fitzpatrick (the artist behind that famous Che Guevara image) has made a living out of creating fantastically illustrated versions of the Mythological Cycle.
  
  
  There are literally thousands of collections of Irish folklore, most of which are decent enough. Original collections by W.B.Yeats and Lady Wilde are also available online

Bit late - I'd suggest Jim Fitzpatrick's books as a solid introduction. He organizes sources into a consolidated re-telling of the story, sometimes characters are coalesced into one but the original sources can have a million different characters doing slightly similar things. The artwork that accompanies the story-telling is fantastic. That book I referenced re-tells how the sidhe, AKA Tuatha Dé Danann, originally came to Ireland.