You will meet her the first time during the quest A Gaelic Alliance in which you are tasked find out why the king of the Picts has lost contact with Donncoirce, the king of Dal Raita. When you enter the village, you find out that the homestead was attacked, but Fingualla managed to escape from the raiding vikings, but her father got captued by them.
She requests from the thegn of Skjern that they try to free him from the vikings before she is willing to go to Perth. After reuniting her with her father, she joins the party until the party arrives at Perth and thus that town will be off-limits if she is to accompany the hird a long time.
Unlike most temporary party members, Fingualla acts more like a recruitable party member. She can be given equipment, you can invest skill points to improve her skills and she may even enter combat if you desire so. Due to her terrible low stats (14 in total), she lacks access to most weapon, offensive and support skills, so it's not advised to let her fight. But she can become a decent camp mother nonetheless.
Fingualla ingen Donncoirce - Default Stats and Equipment