It doesn't help that we have a sort of confused notion of irony thanks to one Miss Alanis Morrisette. As I pointed out in a recent write-up on Jurassic Park, much of our learning comes from elements of pop culture and those frequently get their subjects wrong. Morrisette is more of the rule than the exception--her song Ironic gives a litany of supposedly ironic events, few of which happen to be examples of irony (which is, oddly enough, something of an irony). College Humor recently parodied the song, inserting phrases to make the situations actually ironic (e.g., "It's like rain on your wedding day to the Egyptian sun God Ra").
Most of what we consider "ironic" is actually "tragic" (this has become a running gag on Castle--thanks again, pop culture). But now that we know what irony isn't, what is it? That's harder to suss out than it should be. The concept of irony dates back to ancient Greece, and like many ancient concepts (e.g., humor or love), it's a bit nebulous because it's a single word used to wrap up many loosely related ideas. But a succinct working definition might be "the disparity between expectation and result". How does that help us vis-a-vis David Bowie?
Not much. Unfortunately, the problem is expectation. It sounds dated because it is dated, and it would not be reasonable to expect a song to never sound dated. On the other hand, you might expect a producer working on a song called Modern Love to make an effort to give it a more timeless feel--Tom Petty's You Got Lucky and Every Breath You Take by the Police both came out that year and neither felt quite as of-the-moment as Bowie's contributions. Then again, of the 100 top hits of 1983, I only recognized 3 that, in retrospect, don't sound like songs from the early 80's.
So is it ironic? Well, sort of, but not really. Which, in retrospect, makes this entire post deeply, deeply uninteresting.
C'est la vie,