We don't really hear all that much about Sirens anymore. But for those of us who are vaguely familiar with Greek mythology or The Odyssey, the name might ring a bell. So we may need to refresh our memory with a quick Google search to get the gist of what the speaker means by "Siren Song," but for most of us, we get that we're talking about an allusion to sexy bird-ladies singing alluring songs.
"Siren Song" is also a kind of archetype (original model) for the ideas we associate with seductive villains who may appear as if they need some help or are innocent. How can anyone resist that enchanting song that's actually a "cry for help"? So the title also gives us a little sneak peek into the poem's themes that explore the problems with assuming who's the predator and who's the prey. Along those same lines, the ambiguity of the title helps to pulls us in. When we first get started, it seems like the speaker is telling us about the Siren song. By the end, though, we realize (too late) that the poem's title is not what this poem is about, it is what this poem is. We were listening to the Siren song all along.
Alternatively, the title may get us thinking about how men in particular can be easily fooled into playing the hero and falling victim to the very thing that they sought to save (namely pretty women). Remember: the Siren myth has been around for a mighty long time. Homer is thought to have lived around the seventh or eighth century BC and the Siren myth was around for some time before that, so you get the idea. So, the title itself has been around, but Atwood gives this classic a way more modern spin.