Reddit mentions of The Apple Branch: A Path to Celtic Ritual

For a reconstructionist's approach, I've heard good things about The Apple Branch: A Path to Celtic Ritual.

Since you said you're Druidic, I would also recommend The Druidy Handbook by JM Greer. It provides suggestions for how to open and close a grove as well as celebrate each station of the Wheel Year.

Some common elements you'll find in Celtic ritual include circling the ritual space three times sunwise, purifying the space by burning herbs or incense, and making offerings to the ancestors, aes sídhe, land spirits, or gods. Celtic rituals are also presumed to have taken place primarily outdoors.

Note: I would not recommend approaching the aes sídhe or gods without doing thorough research beforehand.

Druidry often blends these practices with Golden Dawn/Masonic ritual and Arthurian themes. For example, my Druid order AODA (which is headed by JM Greer) circles the ritual space four times sunwise so you can bless the space with each of the four elements (earth, water, fire, air). Those four elements are not attested in Celtic lore that I'm aware of. As far as I can tell, that aspect is derived from Golden Dawn ritual.

Additionally, AODA calls peace in each of the four quarters by drawing and sheathing a sword that represents Excalibur. Calling for peace in the quarters isn't traditionally Celtic, but the incorporation of symbols from Celtic lore clearly is.