I am trying to find a book or other reference to pagan practices in Ireland and Scotland prior to the arrival of Christianity. What I've found deals with the Druids, I want to know how the common man worshipped the gods, goddesses and nature.

As far as the Druids and Pagans of ancient Scotland and Ireland, you will not find many resources for information. Much of what you will find today are reconstructed traditions of what others believe may have been the older Pagans' traditions. Druids/Pagans were not known to record their teachings. Such things were passed down from one individual to another, and so on. You will find their culture/religion hinted at, however, in books, songs, legend, and lore. But you have to keep in mind that these traditions were not written, they were taught. It will be very difficult to find any historic writings about them. And even if you do, you may not be sure of the accuracy of them. I believe most of what was recorded about the Pagans/Druids were burned and trashed by the Romans in their attempt to eradicate the Pagans and reform them into Christians.

Buckland's Scottish witchcraft has nothing to do with ancient Celtic polytheism. For information about the religion as practiced by ancient Celtic peoples, google Celtic Reconstructionism. You'll find accurate information on Reconstructionist sites. Above all, avoid Wiccan sources. Wicca has zero to do with Celtic polytheism.