Ná séit oíbell cen atúd.

Ná séit oíbell cen atúd.

(not * blow * ember * without * (power of) kindling)

Do not blow on an ember that has no fire in it.

This is a more pleasant equivalent of "Don't flog a dead horse." The proverb is preserved in "Iomarbhágh na bhFileadh" (The Contention of the Bards) as "Ná séid aoibhil gan fhadúdh," and is quoted in DIL s.v. "atúd" and "séitid".