Brewer's: Emerald Isle
Ireland. This term was first used by Dr. Drennan (1754-1820), in the poem called Erin. Of course, it refers to the bright green verdure of the island.
“An emerald set in the ring of the sea.”
Nor one feeling of vengeance presume to defile The cause or the men of the Emerald Isle.
E. J. Drennan: Erin.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894