Tristan and Iseult, or Isolde as it is in some versions, is one of the most interesting Celtic stories. For anyone who is familiar with it, do you think it influenced the story of King Arthur and Geneviere?
It might have. But it could also have influenced the story of Lancelot and Guinevere, since both stories are about forbidden love and a cukolded king.
I haven't read a lot about the story but I do think it is probably more closely tied to Lancelot and Guinevere for the reasons that Legend stated. Their love was forbidden and Iseult was the wife of the king.
The story of Tristan and Iseult is actually found as a part of Arthurian legend, and is included in Sir Thomas Malory's Le Morte D'Arthur.
Yes, it is a large part of Le Morte D'Arthur. Tristan is second only to Lancelot as the greatest of Arthur's knights.