The primary sources for these legends are the old books of Ireland where the ancient legends and sagas and stories of Gods and Heroes can be found. They were the folklore of the time and written by the monks of old. 
The partial remains of the Ancient Irish Manuscripts, such as the following, still survive today and are housed in our great libraries: 
“The Book of the Dun Cow”:  
Leabhar na hUidre from Clonmacnoise, circa 1000ad, and now preserved in the Royal Irish Academy in Dublin, contains the great epic of the Táin  Bó Cuailnge 

“The Book of Leinster”:  
Leabhar na Nuachongbala, contains information on the pre-Christian Ireland, and the Táin Bó Cuailgne, Fionn Mac Cumhaill and the Fianna 
and a  list of the Irish Kings is now in Trinity College, Dublin. 
 
 

The Book of Lismore”: 
Contains stories of Conor Mac Nessa and Cu Cúchulainn of Ulster as well as stories of Fionn and the Fianna.