The only song I can find with "Emerald gem of the sea" is Old Ireland Free near the bottom of the page - only one verse though.

They say we are going to freedom
Though my poor heart it lies low
How the English they deceive us
You'll never find a better land
Show me the land to compare with us?
Emerald gem of the sea
I'll cling to my home in old Ireland
Don't I wish old Ireland was free