I don't know about the irony part, but I can somewhat explain the natural selection part, since I see some comment about this already. Natural selection involves bio diversity, a selection pressure, and some inheritance property. Obviously, two fish can't demonstrate bio diversity other than a symbolic representation of 'differences' (the trait being size, in this case). The selection pressure is that bigger organisms tend to be more successful at surviving (not being eaten) than its smaller counterparts. Inheritance is of course missing from the picture, but it's not a stretch to say that big fish tend to beget big fish. So, with all the caveats mentioned, the resulting image could be a symbol for natural selection, abeit an oversimplified one. Given that, then, having a symbol that represents natural selection, which is one of the integral components of the larger theory of evolution, with the attempt to advocate for creationism, is ironic.

EDIT: I must add... I didn't mean that the bigger fish is more evolutionarily advantageous than the smaller fish just because it's able to consume the smaller one. I'm just saying that the selection pressure as depicted in this picture is such. In nature, ironically it's usually the smaller fish that out live the bigger ones as a species.