A House Martell foi fundada por Morgan Martell, conhecido como Andal.
And on the eastern shore, between the Broken Arm and the Greenblood, an Andal adventurer named Morgan Martell and his kin descended on lands loosely held by House Wade and House Shell, defeated them in battle, seized their villages, burned their castles, and established dominion over a strip of stony coastlands fifty leagues long and ten leagues wide.
Over the centuries that followed, their strength grew...but slowly, for then and now the lords of House Martell were renowned for their caution. Until the coming of Nymeria, no Dornishman would ever have counted them amongst the great powers of the country. Indeed, though surrounded by kings on every side, the Martells themselves never presumed to claim that title, and at certain points in their history, they willingly bent the knees to the Jordayne kings of the Tor, the pious Allyrions of Godsgrace, the many petty kings of the Greenblood, and the mighty Yronwoods of Yronwood.
The World of Ice and Fire, Dorne: The Andals Arrive
Como Mors Martell é um descendente de Morgan e não encontro nenhuma referência a eles se casarem fora de Andals até que Mors se case com Nymeria, acho que é provável que ele fosse um Andal.