The King's Druid

The Chronicle of Ireland reports that in the mid-sixth century Fráechán mac Tenusán cast the "druidical fence" over the army of High King Díarmait mac Cerbaill at the battle of Cul Dreimne, and that Díarmait lost that battle due to "the prayers of Colum Cille" (Saint Columba). But how could there have been a practicing Druid in the King of Tara's court in the Age of the Saints, a century after Saint Patrick brought Christianity to Ireland?