According to linguist Michael Everson,
the "Charm of Making" that Merlin speaks to invoke the dragon is
evidently an invention, there being no known classical source. It is
apparently based upon Old Irish.^{[10][11][12][13]}

Transcribed phonetically as spoken in the film, the charm is pronounced [aˈnaːl naθˈrax, uːrθ vaːs beˈθud, doxˈjeːl ˈdjenveː]. The most likely interpretation of the spoken words, supposedly composed in Old Irish, though the pronunciation in the film has little relation to how the text would actually be pronounced in Irish, is:

Anál nathrach,

orth’ bháis’s bethad,

do chél dénmha

In modern English, this would be translated as:

Serpent's breath,

charm of death and life,

thy omen of making.