Aine-Marbh (Aine meaning "brightness" or "joy," and Marbh meaning "death" or "corpse," creating a paradoxical name suggesting undying life or a spirit that transcends death). This uses the concept of "undying" rather than a direct translation, as a direct translation wouldn't sound naturally Celtic.

Alternatively, consider these options, each playing with different aspects of "undying":

• Saoirse-Buan: Saoirse (meaning "freedom" or "liberty") paired with Buan (meaning "lasting" or "enduring"). This suggests an enduring freedom, a spirit that cannot be contained.

• Eimear-Sir: Eimear (a name meaning "pleasant" or "beautiful") combined with Sir (meaning "everlasting" or "eternal"). This creates a name suggesting lasting beauty or an eternal pleasantness.

• Fionnuala-Mara: Fionnuala (meaning "fair shoulder" or "white shoulder," often associated with mythical figures) paired with Mara (meaning "sea"). The sea is often associated with immortality or cyclical renewal in Celtic mythology, suggesting an undying connection to nature.

• Aisling-Buan: Aisling (meaning "dream" or "vision") paired with Buan (meaning "lasting" or "enduring"). This suggests a lasting dream or vision, an undying hope or aspiration.

These names attempt to capture the essence of "undying" within a Celtic naming convention, using existing Celtic words and names to create something both evocative and authentic-sounding. The best choice will depend on the specific nuance you wish to convey.