(Someone in this Reddit thread claimed to have worked with her on the Season 1 set. She's uncredited, but the woman in a white bonnet and a mustardy-brown cloak is most definitely her.

The Highlanders camped around Castle Leoch watch on and one woman in the crowd is none other than Iona Claire, who would go on to play Fiona in Outlander Seasons 2, 3, and 4. In Season 1, Episode 4, 'The Gathering,' Caitriona Balfe's Claire is walking behind the dead body of Geordie, who was just impaled by a boar during a hunt. (Though it's probably not the third option.) But let's take a trip back to when the actor behind Fiona appeared in the 18th century. Fiona was in Outlander Season 1 - well, at least the actor who plays Fiona, Iona Claire - as a person attending the gathering at Castle Leoch in 1743, and whether it was an Outlander mistake, some inspired casting (think Graham McTavish as Dougal's son Buck), or a clue to an outrageous storyline, it's not entirely clear. But one character whose presence can't be explained in the 18th century is Fiona Graham. When you're a time traveler like Claire on Outlander, people can pop back into your life rather unexpectedly and in unexpected centuries.

Outlander season 1 episode 4