What does the color green for Ireland stand for?

Green symbolizes:

• The shamrock (which, because of it's three leaves in itself symbolizes the Trinity)
• One of the names for Ireland: "The Emerald Isle"
• The season of the year when plants are beginning to show green

It is said that St Patrick used the shamrock leaf to illustrate the conception of the Trinity as three apparently separate entities being one thing and this legend has given rise to the adoption of the shamrock (literally, "little clover") as the national symbol of Ireland and, probably too, as the colour representing the original inhabitants of Ireland.

The poetic name 'The Emerald Isle' (emerald being a green precious stone) may come partly from the appearance of Ireland from the sea - compare with one of the old names for Britain, Albion, meaning "the white", from the white chalk cliffs of Dover - but may also be because the colour of the shamrock was adopted as a national colour.

It is also sometimes said that leprechauns, in particular, wear green (perhaps so that they can vanish from view more easily) but this is likely to be more of a reinforcement of the fact that leprechauns are peculiarly Irish and therefore wear the colour appropriate to their country.