Moody's said its decision was based on the "growing possibility" that Ireland would need a second bail-out before it can return to capital markets.
The current European Union and International Monetary Fund support programme is due to end in late 2013.
It comes at a time when markets fear the debt crisis in the eurozone could spread to Italy and Spain.
Ireland, Greece and Portugal have all been downgraded by ratings agencies several times in recent months.
THE EURO CURRENCY WAS A BAD IDEA DREAMT UP NY THE ELITISTS EURO-LEFT AND IT WILL BE A GOOD THING WHEN IT COLLAPSES AND DISAPPEARS.
IN THE MEANTIME: TRADE YOUR EUROS FOR GOLD OR ... GULP: DOLLARS.