Gaelic society had no runes but early on used ogham, mainly for dedications on upright stones. Ogham often appears on the edges of stones, the line representing the edge.

What language did Druids use?

The vast majority of druids spoke Drueidan; those from the Moonshaes spoke a language called Daelic.

What were Celtic runes used for?

There is no certain evidence of wide literary use of runes in early times, but some scholars hold that the runic writing was widely employed for all kinds of secular documents, such as legal provisions, contracts, genealogies, and poems.