As of yet, the individual cell is the prime contender to be an irreducibly complex system, however biochemistry and micro-biology is not advanced enough to be able to list and funny understand every part of a cell. Therefore, the hypothesis is unprovable as of yet, but it is testable.
Ok so we have a hypothesis here, and correct me if I'm wrong but the logic goes something like this.
1. An irreducibly complex system is one that needs all of its parts fully developed to have any function, if one part wasn't there, it would be useless and not favorable for the propagation of genes.
2. Irreducibly complexity therefor can not be brought about by evolution.
3. Therefor if this irreducibly complex system could not have evolved, it must have been designed by some kind of designer.
It is not debated that some mutations, or evolution, can occur over time.
Ok we've got that out of the way.
Now you noted that is not provable, but is testable. What kinds of tests are being done on what seems to be the sole hypothesis made by proponents of ID? Most of the literature I've seen from people pushing ID and creationism from places like the ICR does not address a hypothesis about ID at all, many of the articles are things like how Darwin was responsible for the holocaust or that evolution is dangerous and makes kids kill their class mates, usually invoking Godwin's fallacy. (Hitler did/believed it, so it's bad).
I'll spare you the run down on all the different cells that can detect the visible spectrum, and other wavelengths of EMR. Because that got really long and was going off topic but I think it kind of shows the problem where the ID/evolution debate breaks down. An ID proponent says "according to my work, it would appear that evolution can't produce X." Then an evolutionist does a bunch of testing and says "wait a minute, actually I've found evidence that it can." Which is followed by the naming of something that they claim evolution can't bring about. So we're left with a growing stack of evidence in favor of evolution, and a dwindling one for ID. That cycle will probably go on for decades to come unless there is another hypothesis formulated from ID to test.
So if I'm following this right, if one takes into consideration the evidence for evolution, and then accepts the irreducible complexity arguments for ID, then what you have is pretty much micro/macro evolution intact, but it was all set in motion when some designer made some cells with those parts deemed irreducible. Is this what ID is asserting? that Irreducible parts must have been designed, and the rest that looks to have evolved wasn't?
But on a completely different note, on theology instead of science. Does the above not represent a "god of the gaps" doctrine, rather than anything that is talked about by Christianity (apart from Deism)? Saying "Ok here's something we can't explain, god must have done it." Is that even reconcilable with the major western religions that take genesis to have any kind of face value on a creation? To put it in overly simplistic terms, the ID argument of irreducible complexity in a Christian context would seem to say "god created adam, a cell with a photosensitive organelle and a flagellum, and then he evolved into what we see today." Christian/Jewish/Muslim doctrine seems to be in direct disagreement with whole god of the gaps line of thinking. So I have to ask, does anyone here actually believe what this sole hypothesis from ID would suggest, that these few items deemed irreducible were created, and everything else evolved from that? Because almost every time I see someone take issue with evolution, it's an assertion that man didn't branch out from apes, we've always been like we are etc. Not that "the cell is too complex to have evolved, god must have made, but i agree that the fossil record and genetics do show substantial evidence that we evolved from simpler life forms."
Also given this line of thinking, it appears that ID does not make any hypotheses to dispute micro evolution, macro evolution (99.999% of it), or the formation of the universe, only abiogenesis and how we arrived at those very basic systems deemed irreducible. Is this correct?