The Sand Snakes are mostly in A Feast For Crows. They only show up in one chapter of Dance With Dragons. But the Dorne storyline is one of the better parts of the AFFC. The Dornish are also important in Dance with the whole Quentyn Martell storyline, but that one has it's ups and downs.

You know, I've been re-reading the fourth and fifth books recently, and I feel like the AFFC gets unfairly shit on. Yeah, the Aeron chapters are a fucking drag. Victarion isn't that interesting. And the Brienne and Jamie storylines take a little while to get going. But by the end, the latter two are among the best parts of the book. And, especially in the Brienne story, they touch on a part of the story that the TV show almost completely ignores: the cost of war on the common people.

While the great houses are playing their Game of Thrones and fighting over the Iron Throne, the people are suffering, starving, and dying. It's partly why the High Sparrow becomes so popular. And it also shows that when Winter really does come across all of Westeros, everyone is screwed. While everyone has been fighting over the Iron Throne, no one has been preparing for Winter. No one has been growing and storing crops. A lot is made of the Lannister army essentially burning the fields of the Riverlands; I imagine it's the same everywhere. So the Seven Kingdoms are not going to be close to ready whenever the Long Winter REALLY begins and the White Walkers do breach the Wall and rain their terror on everyone.

I do agree that a lot of the fifth book is pretty tedious. The Dany story just isn't interesting. The Tyrion stuff only sort of. Most of the good stuff is with Jon and Theon/Reek.