Dario, pronounced DAHR-ee-oh, is the masculine Italian form of the Latin Darius and Greek Dareios. Ultimately it comes from Persian name Dārayavahush, simplified to Dariush, meaning "he who holds firm the good," or "to possess good, to possess well" but is sometimes taken to mean "preserving good," "upholding good," or "protector." The English name Darien (#1000 in 2013) can be either from the Latin or Greek form, or from the questionable Irish Gaelic name Darren, meaning "great."
Three ancient Persian kings had this name - Darius I, Darius II, and Darius III, the first of which was Darius the Great. Rulers Darius I and Darius II of Media Atropatene also shared the name. Prince Darius the son of King Mithridates VI of Pontus is another, and his father claimed he was a descendant of Darius the Great and/or Cyrus the Great, and Mithridates also had a grandson named Darius of Pontus, though he was a child of Mithridates's son Pharnaces. Dara Shikoh (sometimes Dara Shukoh) was an almost-Emperor of the Mughal Empire. Darius the Mede comes from the Book of Daniel and was said to have been king over the Chaldeans. Lastly, there was a Darius who was a politician in the Eastern Roman Empire.
Saint Darius (Saint Dario) was martyred in Nicaea, Turkey in the 4th century with Zosimus, Paul and Secundus.
For the spelling Dario, namesakes include Nobel Prize winner for literature Dario Fo, film makers Dario Marianelli and Dario Argento (father of actress Asia Argento), Italian poet Dario Belleza, soccer athletes Dario Šimić and Dario Vidošić, and Olympic gold medalist Dario Cologna all share the given name, but there are many more.
For the spelling Darius, namesakes include several athletes, musician Darius Rucker, cinematographer Darius Khondji, French composer Darius Milhaud and others less known. It has been used very little in fiction, and mostly recent.
Darien has the fewest namesakes and fictional characters - there are two actors, a journalist and obscure TV, game and books appearances.
In the mid-20th century this name was used more commonly. Today it ranks at #986 in the U.S. while Darius ranks at #412. Dario currently ranks in Croatia and Spain, and has been used well in Galicia and the Netherlands.
The international female version is Daria. "Daria" was a hit animated TV show from the 90's with a lead character of the same name. Saint Daria was martyred with her husband Chrysanthus in the 3rd century. Alternate spellings for Daria include Dariya, Darija, Dareia, Darya, and the lesser known international options Tarja, Darina, Darinka, Darushka, Derya, Daryna and Odarka. Daria currently ranks in Poland, England and Wales, and was last ranked in the U.S. in the late 90's. Dasha/Dasia are Russian, Ukranian and Polish nicknames. Daryā also means "sea" in modern Persian. Daria has more namesakes than all of the masculine versions of this name, most of which are atheletes.