They're only ever mentioned briefly in the comics, usually only in the context of having their own El world -- the World of Elemental Creatures like Elves and Goblins.
Now, we have seen Goblins in the Slaine universe several times, and they're detailed in the rulebook. So one might assume that Elves are similar -- basically not all that magical. However, Elementals are the wild and weird things that are covered in the PDF download -- very different to Goblins. Then there's fairies, covered in the Slaine series "The Secret Commonwealth" (don't know the prog numbers off-hand). These were mostly pretty unpleasant, and not entirely dissimilar to Elementals. Finally there's your more general Els, but they come from the World of Dev-Els and Ang-Els.
Any or all of the above might be seen as representing 'Elves' but on balance I decided not to make a firm decistion about which if any of them to call 'Elves.' Basically the more Tolkienesque or Norse-style elf isn't reallly a feature of the Slaine setting, and said setting isn't really sufficiently strictly defined to say for sure which of its denizens might be classed as an elf. I guess characters might call any vaguely humanoid El creature an elf, if they wished, but there isn't an official elf as such.