Their name comes from the iron masks they wear to protect their identity while on the assignments (and to make them look more intimidating).
They operate almost exclusively within the city, and are known for their "no questions asked" policy, as well as taking assignments no other similar agency is willing to undertake. This means that many of their assignments involve "dirty" dealings, but the company has iron clad contracts that absolve them from responsibility for any wrong doings on behalf of their client.
The truth is that Iron Guard is connected to Tor-Ban criminal syndicate, and operate under the purview of the Shadow Council. This means that they have various profit sharing agreements with the other council run extortion gangs. They are able to maintain stellar track record at virtually no risk by sharing portion of their earnings with those whom they are hired to fight off.