Ancient Irish rhetoric has become a rather consuming passion of mine recently. Very little has been done on the subject, as far as I can tell. So, there is plenty of room for exploration.
Recently, Paul Lynch and I completed an article on the rhetoric of the ancient Celts, including issues of Druidism, and St. Patrick. You can download the article as a PDF with the link below:
Rhetoric of Myth, Magic, and Conversion: Ancient Irish Rhetoric
Here is a slide version for people who don't like to read so much:
Myth, Magic, and Conversion: The Rhetoric of St. Patrick