c. 1200, amiral, admirail, "Saracen commander or chieftain," from Old French amirail (12c.) "Saracen military commander; any military commander," ultimately from medieval Arabic amir "military commander," probably via Medieval Latin use of the word for "Muslim military leader."
Amiral de la mer "commander of a fleet of ships" is in late 13c. Anglo-French documents. Meaning "highest-ranking naval officer" in English is from early 15c. The extension of the word's meaning from "commander on land" to "commander at sea" likely began in 12c. Sicily with Medieval Latin amiratus and then spread to the continent, but the word also continued to mean "Muslim military commander" in Europe in the Middle Ages. The Arabic word was later Englished as emir.
As amīr is constantly followed by -al- in all such titles, amīr-al- was naturally assumed by Christian writers as a substantive word, and variously Latinized .... [OED]
Also in Old French and Middle English further conformed to familiar patterns as amirauld, amiraunt. The unetymological -d- probably is from influence of Latin ad-mirabilis (see admire). Italian form almiraglio, Spanish almirante are from confusion with Arabic words in al-. As the name of a type of butterfly from 1720, according to OED possibly a corruption of admirable.