Sean or Seán is an Irish derivative of the Norman-French forename, Jehan, which itself derives from the Latin "Joannes." Like many other common Irish forenames (Sinéad, Piaras, Siobhán, Liam, etc.) Seán first appears in Ireland following the Norman invasion of Ireland. Common variants of the name include Shaun (chiefly British), Shawn (chiefly USA), Ceon (Scottish) and Shane.

With its Latin origin, Seán has a long history that traces back to Hebrew name יוחנן (yôḥānnān), which means "Yahweh is gracious". Yahweh is treated as the name of God in Hebrew. Yôḥānnān was the name of several important Jewish rabbis in the Second Temple Period in Palestine, such as Yochanan ben Zakai and Yochanan ben Nuri.

The name had gained popularity among Jews in Judea and Galilee by the time the area became a province of the Roman Empire in 6 A.D. It was the given name of Yochanan ben Zechariah, a Jewish prophet known in English as John the Baptist. It was also the given name of Yochanan ben Zibhdi, a fisherman from Galilee who became one of the favorite students (called disciples) of Jesus Christ and so is known in English as John the Apostle. Because Yochanan also wrote one of the four accounts of the life of Jesus Christ called gospels, the Gospel of St. John, he is also known as John the Evangelist.

The texts that tell of the lives of both these men named Yochanan were written in Greek, and their name was adapted in Greek as Ἰωάννης, Iōánnēs (pronounced YO-han-NAYS). The name Ioannes became extremely popular among the early Christians, and bearers include such noted members of the early church as Ioannes Chrysostomos and the Ioannes who wrote the Book of Revelation.

Because of the great respect Christians had for these men, the name came into use in other parts of the Christianized Roman Empire, even in remoter parts such as Gaul and Britain. The Western areas of the Roman Empire did not, however, speak Greek like the areas in the East. Instead, they spoke Latin. Accordingly, in the Western part of the Roman Empire the name was Latinized as Iohannes (pronounced like the Greek).

The local populations in these areas of the Roman Empire soon changed Roman names to fit their own dialect, which included dropping the suffixes -us and -es from such names. Johannes became the Germanic Johann, for example, and on the outskirts of the Empire in the newly converted Ireland it became the Irish Eoin. In some cases, the pronunciation of the "J" also changed from the original "Y", so that in Iberia the name eventually changed to the Spanish Juan and in Gaul to the French Jean.

In the 11th century the French duke William the Conqueror invaded and conquered England and brought his French knights and their dialect with him. In England, the French name Jean came to be spelled John. The Norman kings of England also conquered Ireland in the 13th century and the 14th century. The Irish nobility was replaced by Norman nobles, some of whom bore the French name Jean or the Anglicized name John. The Irish adapted the name to their own pronunciation and spelling, producing the name Seán. Sean is mostly pronounced SHAWN, but in the northern parts of Ireland it is pronounced SHAYN, thus leading to the variant Shane.