While the author, Christopher Paolini, drew inspiration from various sources for the language, including Celtic languages like Gaelic, it is not a direct translation or adaptation of Gaelic.
Paolini has stated that he created the Ancient Language specifically for the world of Eragon, drawing from a variety of sources, including Latin, Old Norse, and even invented elements.
While the Ancient Language shares some similarities with Celtic languages in terms of its sound and structure, it is ultimately a unique and distinct language designed for the fictional world of Alagaësia.