Generation I DS:
1) Natsu
2) Gajeel
3) Wendy
Generation II DS:
4) Laxus
5) Cobra
Generation III DS:
6) Sting
7) Rogue
Serena would be classified as a GII, though it could be argued that he belongs in a separate generation because he has multiple dragon lachrima inside of him. However considering Acnologia started counting from the moment after he already beat him, it doesn't really matter much.
There was never actually any discussion to whom Acnologia was referring originally, however now that Irene has been introduced some people became confused. Certain people claim that it's a plothole and that Hiro only developed Irene's background after Acnnologia made that remark. That's not impossible as she indeed feels as being added rather on the spur of the moment, but there are still very well alternatives.
For starters technically speaking Irene is not a DS, but a dragon and unlike Acnologia she didn't had the ability to change between her human and dragon form. It was Zeref who turned her back into a human and till further notice she hasn't shown the ability to turn back nor any abilities that are associated with DS (yet). So she's really an unusual case, which might be the reason Acnologia couldn't sense her as I seriously doubt the latter first spend some time investigating who exactly are all the dragon slayers. He's most likely using some kind of sensing ability. You can't ignore either that Irene is an extremely powerful mage and enchantress and that she is somehow able to camouflage herself. Don't forget that the whole point of this war is to defeat Acnologia and that she might have thought it would be better to not alarm him of her constitution.
You can also reason that Irene was simply out of his range to be sensed. The 'battlefield' was after all an entire country where they were fighting on multiple fronts. Though if this was the case, then it would be quite weird that he was able to sense Cobra, but not Irene. The Oracion Seis arrived quite late, but it still would be possible depending on where they exactly were.
In any case I think we can safely discard the possibility that there's still an unidentified DS running around.
