Greek mythology predates Celtic mythology. Greek mythology has its roots in the 2nd millennium BCE, with the earliest written records dating back to around 700 BCE, such as the works of Homer and Hesiod. These texts provide detailed accounts of Greek gods, heroes, and mythological creatures.
On the other hand, Celtic mythology, which includes the myths of the Celtic people of Western Europe, was primarily an oral tradition until much later. The earliest written records of Celtic mythology date back to the early medieval period, around the 5th to 6th centuries CE, when Christian monks began to document these stories.
Therefore, Greek mythology is older than Celtic mythology by several centuries.