The leader of Dorne is Prince Doran Martell, Oberyn's paramour is Ellaria Sand, a bastard of House Uller. The Martell's had a major stake in the War of the Usurper. Doran and Oberyn's uncle was a member of Aerys' Kingsguard and fell at the Trident, and of course their sister Elia was married to Rhaegar and had two children with him, their son Aegon being the heir once Rhaegar ascended to the throne. Then he abandoned them more or less and spirited away Lyanna, and ultimately all three were murdered and Elia raped by the Mountain on Tywin's orders during the sack, leading to Oberyn fighting in Tyrion's trial for revenge and admission, and dying. Cersei sure ain't safe with former Targaryen loyalists to her south, especially the ones with most cause to despise the Lannisters for 20+ years.