well it is irreversible - if it melts and the water flows in the ocean and mixes with the water there, there is no way to put it back. Of course if it cools down over the next few million years Greenland can grow a new glacier. But that will be a different one.
Given this article, it might be less irreversible then previously thought... but the CO2 will have to be removed from the atmosphere.
This argument is a bit like the "coal is a renewable form of energy because it eventually returns" argument. The consensus is that if the time scales required are on the order of thousands, tens of thousands, or millions of years, then it's safe to call it nonrenewable, and it is safe to call this irreversible on a human time scale.
Coal exists because microorganisms and fungi capable of breaking down lignin didn't exist then. It will never be made again on Earth.