I believe Ys I and II were released (separately) on the NES as well. As Rune mentioned, there was a Japan-only remake on PC, Ys I and II Eternal, and this was later ported to PS2 as Ys I and II Eternal Story. I was quite diappointed that WD didn't pick it up and release it as Ys I and II Eternal Story Complete.
That is very true, Alunissage. To date, Ys has been released on nearly every major console. Ys I and II ("Ancient Ys Vanished: Omens" and "Ancient Ys VanisheD: The Final Chapter" respectively), comprise the first part of the series, taking place in Esteria (which, BTW, was inspired by the real-life offshore island of Mont-Ste.-Michel in France).
Weirdly enough, the two versions of Ys IV ("The Dawn of Ys" for PC Engine, and "Mask of the Sun" for SNES), set in the land of Celceta, takes place BEFORE the events of Ys III!
Continuing on that note Ys III ("Wanderers from Ys" for Sega Saturn and its later remake "The Oath of Felghana"), take place in the region of Felghana, some six to nine months after the Celceta adventures.
Ys V (subtitled "Lost Sand Kingdom of Kefin") takes place in Xandria not long after that.
Then, the series proper concludes with Ys VI: The Ark of Napishtim, which takes place three years after Adol's adventures in Xandria. The events of this game also bring the series full circle, answering some unasked questions.
For instance, some long-time fans will realize that the Emelas seen in Ys VI is actually CLERIA, which played a large part in Ys I, II and IV! Another example: Alma's race, the Eldeen, are connected not only to the Celceta Dynasty of Ys IV, but also the Kingdom of Ys from the first two games...which opens up the possibility that the twin Goddeses of Ys, Feena and Reah, may in fact ALSO be Eldeen!