In the television show Stargate SG-1, the iris is Earth's first line of defence against an attack through the Stargate by hostiles. Originally composed of a large number of titanium plates housed in the outer ring of the Stargate, the iris slides closed over the event horizon in the event of unauthorised incoming 'gate activity.
The iris is situated a mere few micrometers from the event horizon, meaning that matter travelling through an incoming wormhole and encountering the iris is prevented from fully reintegrating, causing it to cease to exist in a reverberating thud. This rather final end happens to all objects impacting the iris, whether they be living creatures (humans, aliens and other space-related things) or solid objects (bombs, rocks, bullets). The only known exceptions to this are radio waves (and likely most of the rest of the EM spectrum capable of passing through solid matter), near-light-speed particles and gravitic fields and their associated temporal distortions.
In order to prevent friendly incoming travellers through the Stargate from ending up as a smear on the iris1, it can be deactivated both automatically by a computer upon receipt of an authorised code sent via radio through the 'gate, or manually using a palm-scanner in the control room. When the iris is deactivated, its component plates slide back into the outer rim of the 'gate.
Following an incident involving a black hole, the Stargate, and the whole host of temporal and gravitic problems associated with such things, the original iris was lost (torn from its housing and sucked through the Stargate to be more specific). This was replaced with a new iris composed of a trinium/titanium composite material, making it much stronger and able to withstand a lot more punishment.
An interesting fact to note about the iris is that when the 'gate is activated with the iris closed, the energy vortex normally produced upon 'gate activation is contained behind the iris. This is likely due to the proximity of the iris to the event horizon - normally the energy vortex tends to vaporise anything in its path before it settles down and the wormhole stabilises.
1 - Please note that things impacting the iris after travelling through the Stargate do not actually smear across it. They are prevented from reintegrating fully. This means that when they impact the iris, they are only partially 'there' - ie: only a part of their matter has appeared, so very little, if anything, is left as a residue on the iris.