I also tend to see it as more of an earnest horror film than an ironic deconstruction of the genre if only because the horror elements a) stray far from cliched horror conventions (e.g. the scariest shots in the flick are slow, static, and technically rigid instead of the quick, startling, and raw sequences in most horror), and b) are genuinely terrifying in an "Oh crap I'm going to have nightmares about this" sort of way.
I think that the film's most explicit horror elements (the blood gushing out of the elevator in slow motion, the successively looming shoots of the little girls intercut with the aftermath of the "slasheresque" violence done to them, REDRUM/MURDER, etc.) exaggerate, and therefore render ironic, those aspects that might be found in a more conventional horror film. The genius of Kubrick is that he allows these elements not only to refer to the horror genre that he is, in a way, parodying, but also to become signifiers the sublime in human consciousness (otherwise this film would merely be a witty, meaningless little popcorn film like "Scream"). In other words, I think that he exaggerates certain conventional aspects of the horror genre to such an extent that its essence presents itself. There must be a reason why humans are drawn to the brutal and stylised violence of the slasher film, and cinematic violence in general. Kubrick exposes this by parodying it. The imagery is simultaneously culture bound (in that it refers to the horror film) while also attempting to present something mythic or even transcendent, something implicit, about the sublimity of violence in human consciousness. This is why, I think, the images are so effective (and affective): they are designed to touch on something intrinsic about our violent natures.