‘You will be one with the birds’ was the curse put upon King Sweeney by Bishop Ronan Finn. This story of Mad Sweeney and his cursing by Ronan was written down as late as the seventeenth century, but is often thought to date as far back as the Old Irish period – in fact, it seems to recall an incident with a much broader pedigree. The madness of the king of Ulster, linked in the Frenzy of Sweeney to the battle of Mag Rath of the year 637, has a close reflection in early Welsh recollections of the madness of Merlin. At the sixth-century battle of Arthuret, the Celtic magician par excellence is also supposed to have gone mad, and was likewise left for a time to wander in the wilds. Merlin recovered from his madness, however, seeing out his strange ordeal, one that in the surviving, moreover, won him prophetic powers. A different fate awaited the wretched Sweeney: he spent the rest of his life madly hopping about Ireland and Britain as if he were a bird. Yet the two tales are often thought to be linked – one (perhaps the Welsh) having influenced the genesis of the other. Indeed, it is almost as if the concept of destining a destiny has somehow been inverted in the Welsh Merlin tales: a curse has given its victim prophetic powers instead of such powers being used to destine a curse. How Merlin's madness came about is not clearly explained in the early Merlin poems, however.

The usual approach in medieval literary studies until the 1960s was to focus mainly on how early tales first emerged and developed over time. Nowadays, however, the approach is usually to focus instead on what such tales meant from a day-to-day perspective. Consequently, strange episodes such as these Celtic madnesses are often linked in more recent works with shamanistic practices – rites and rituals which induce altered states of consciousness. Such psychologically transformative experiences are commonly promoted by contemporary anthropologists as representing universal features of magical practice; and, indeed, both ofthe instances of madness suffered by these early Celtic literary figures are suggestive of some of the rituals which are recorded of medieval Finnish and Lappish magicians and seers.