Any book that includes Irish Mythology includes Irish Paganism and you have this prompt covered.  If you don't like mythology and folklore, maybe a book that mentions something related to one of the Celtic festivals?  Note that catholicism maps the timings of its saint's days to these pagan dates and many books that are set around the Christian festivals are set around the pagan ones too.    There are four main Celtic pagan  festivals spread throughout the year.

1.  Samhain - You know this as Halloween - the festival is on the 1st of November and the eve is Halloween 31 October.  The Halloween traditions you know are what is left of the original pagan rites. 
 
2.  Beltane - 1st May.  A celebration of early summer.  So a book set in early summer?  Cattle were also crucial in this festival and there are many books by Irish authors about cows still!

3.  Imbolg - This will be on the 6th of February 2023 and is St Bridgid's day in the catholic calendar.  It is a celebration of the start of spring and the pagan goddess Brigid.  You would traditionally ask Brigid (whether in the form of a goddess or saint) to protect your home that day.  So I guess books about Spring, home, and family would be good here. 

4.  and Lughnasadh.  There is a play called Dancing at Lughnasa (it was made into a movie in 1998).  Lughnasadh is the harvest festival and the most famous one still celebrated is the Puck Fair in Kerry.