Song, which I'm familiar with thanks to my favorite book Howl's Moving Castle by Diana Wynne Jones. The poem's also used in Neil Gaiman's book Stardust.
But, for the drawing itself. Not wild about the pose (my mermaids are still too stiff), but I had a lot of fun stylizing it like that. Incorporating text was a fun experiment as well--I could see trying that more often.
Here's another mermaid I did a while back, also in pen & ink and rather art nouveau. It's silhoutted, mostly in attempt to mimic my fairy drawing (see my profile pic)--I've yet to succeed in creating a silhoutte I like as much as that one (depressing when I created it in 2007--the fairy one, not the mermaid).
I'm not quite sure of the reason for my fascination with drawing mermaids on rocks (I did one a couple of weeks ago on a leftovers box, too--talk about creative medium) but I will admit that my childhood obsession with Disney's The Little Mermaid probably didn't help. Of course, I love the fairy tale just as much if not more, despite its sobering conclusion. (Side note: If you follow my fairy tale links to surlalunefairytales.com, on the lefthand column you can find the link "illustrations" for pictures of artwork for that particular fairy tale by various artists--or you can go straight to the illustration gallery. I've had a lot of fun browsing through those; you may want to check them out.)