I've always added the Irish moss at 15 minuted left in the boil like I was told when I started. But I'm wondering now why 15 mins? what happens if its in for a couple minutes longer or what happens if its in for a whole hour? or on the other side is it effective as a 10 or 5 min addition?
The answer is that the active ingredient, carageenan, is said to denature by hydrolization at low pH (especially if combined with high temps) before it has gelled, but it is hard to find citeable sources on the Internet. Carageenan typically comes from seaweed, including the commonly-named Irish Moss.
Most forms of carageenan are not soluble in water at room temperature, but are soluble at higher tempertures and form a gel at room temperature. Source: Wikipedia.
The active ingredient in Irish Moss is carageenan. The active ingredient in Kerry Bio-Science's Whirlfloc (link to MSDS) and Five-Star Chemicals' Super Moss (link to MSDS) is also carageenan. Whirlfloc comes in many formulations (Whirfloc BWS [powder for 40BBL+ applications], Whirlfloc T [tablet form of Whirfloc BWG], and Whirfloc G [granulated form for commercial food uses]) is marketed mainly to commercial users.
The difference from Irish Moss is that Whirlfloc and Super Moss are products in which the carageenan has been refined and concentrated (from other typeseaweed), and then added with binders and aids to dissolve it in liquid.
The carageenan works as a fining agent in a similar way as gelatin (in fact it is a gelatin substitute for vegetarians who cannot eat gelatin) - by attaching to proteins, which in turn have attached to tannins, and yeast. It gelatinizes around 60°C (140°F) (Source - see section titled "Gelation of carrageenan ").
It is said that the carageenan in Irish Moss, Whirfloc, Super Moss, and similar products needs about five minutes at boiling temperatures to dissolve and gelatinize, but are susceptible to hydrolization if kept at low pH, especially in combination with high temperature. Wort has a low pH. But gelled carageenan becomes stable at room temperature, even at low pH. So here is a source discussing carageenen, it hydrolization, and some of its characteristics: (Source - see items #2 and #4 in particular).
In light of the above info, I am convinced that we should add Irish Moss, Whirfloc, Super Moss, and similar-acting finings at between 10 min. and five min. left in the boil, as per the advice given by several online homebrew shops and in online forums like Homebrew Talk. We want to add with at least 5 minutes to allow gelation, and 10 minutes seems like an outside limit that is cited by many sources as safe.
Adding irish moss too early to the boil will actually cause the coagulated proteins to break apart after binding together, negating the purpose of using irish moss. Ten minutes would be fine, but any shorter and you risk not giving the irish moss sufficient time to bind the proteins together.
As an aside, adding irish moss will help with clarity, but quickly chilling the wort will optimize its use.