First, what you need to understand about the Ironborn (Iron Islanders) is they do not like to be governed by anyone off the islands, especially the Northern lords and the King.
On the Iron Islands, Balon Greyjoy (Yara and Theon's dad) mysteriously dies just as his brother Euron returns home after a long voyage. Balon had two other sons who were killed in the Greyjoy Rebellion that immediately came after Robert's Rebellion. This leaves no true heir to the Seastone Chair, which basically has a built-in obligation to try and win independence for the islands.
What ensues is a Kingsmoot, an Ironborn tradition to determine who will lead them, and the candidates are Euron, Balon's other brother, Victarion, and Yara. Victarion and Yara hate Euron for never being home, for allegedly killing Balon, and for just being a shady guy all-around.
Euron wins by making grand promises and showing off some magical and mysterious items he picked up in his travels, one of which is a horn that is supposed to tame a dragon. He also plots to take Daenerys as a wife in order to conquer Westeros and sends Victarion to capture her.
Thrones does not appear interested in casting Victarion, or at least he will not be a major character in the show. This is where Thrones could continue to re-work the Martin's book plot, by condensing the Iron Islands' list of characters and focusing on a conflict between Yara and her uncle Euron.
But while Whelan confirmed she is back in Belfast for filming, she is staying on her toes to not give anything away.
"Even when I said I'm going to Northern Ireland on Wednesday to you just now, I thought 'oh god, maybe I shouldn't say that.' You really watch your back on everything - 'don't say too much,'" she told British Comedy Guide. "But at least it's been leaked that I'm going to be in it, so I can talk freely about that."
