You could either:
* Use the original name: "Kirsty" is perfectly understandable in French.
* Find a similar-sounding French name: Some names that sound similar to "Kirsty" include "Christine", "Cristine", or "Kirsten".
* Use a translation of the meaning: If "Kirsty" has a specific meaning (e.g., "Christian" in some interpretations), you could find a French name that translates that meaning.
Ultimately, the best option depends on the context and your preference.