The first thing to remember in terms of the actual practices of the Irish Celts is that we actually know almost nothing about them since they had no written langauge. The Celts depended upon an oral tradition and in the 1500-some years that Ireland has been a Christian peoples that oral tradition ha been lost.
I would suggest starting with books on the historical Celts and what little we actually do know. You might want to try these for starters:
"The Pagan Celts" by Anne Ross
"Pagan Religions of the Ancient British Isles" by Ronald Hutton
"The Druids" by Peter Berresford Ellis
There is an excellent site with lots of very good resources at https://www.ecauldron.net/reconcelt.php
As for modern Celtic Reconstructionis path, I would recommend:
"The Apple Branch" by Alexei Kondratiev
And finally, here's a link to a site that contains a multitude of Irish Celtic myths and legends which can give you an idea of how the ancient Irish Celts viewed their Deities: http://www.ancienttexts.org/library/celtic/irish/
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