Dicuilus (or the more vernacular version of the name Dicuil) was an Irish monk and geographer, born during the second half of the AD 8th century.
The exact dates of Dicuil's birth and death are unknown. Of his life nothing is known except that he belonged probably to one of the numerous Irish monasteries of the Frankish Kingdom, became acquainted, by personal observation, with islands near England and Scotland. From 814 and 816 Dicuil taught in one of the schools of Louis the Pious, where he wrote an astronomical work, and during 825 a geographical work.
The astronomical work is a sort of computus of four books, in prose and verse, preserved only in a manuscript which belonged formerly to the monastery of Saint-Amand in northern France, and is now at Valenciennes. 
De mensura Orbis terrae
Better known is the "De mensura Orbis terrae", a summary of geography, giving concise information about various lands. This work was based upon a "Mensuratio orbis" prepared by order of Emperor Theodosius II (AD 435), a manuscript copy of which was possessed by the Carolingian court. Godescalc had already made use of this copy (781-783) for the composition of his celebrated "Evangelistarium". Dicuil uses Pliny the Elder, Gaius Julius Solinus, Paulus Orosius, Isidore of Seville, and other authors, and adds the results of his own investigations.
In the nine sections he treats successively of Europe, Asia, Africa, Egypt, and Ethiopia, the area of the Earth's surface, the five great rivers, certain islands, the length and breadth of the Tyrrhenian Sea, and the six (highest) mountains.
Although mainly a compilation, this work is not without value. Dicuil is our only source for detailed information of the surveys performed by order of Theodosius II; his quotations, generally exact, are of service for the textual criticism of the authors mentioned; of great interest, too, are the few reports which he got from the travellers of his time; as, for instance, from the monk Fidelis who (AD 762?) journeyed along the canal then still existing, between the River Nile and the Red Sea; and from clerics who had visited the Faroe Islands and lived possibly in Iceland for six months.
Based on similarities of style, it is thought Dicuil may be the same person as Hibernicus exul.
The manuscript was known to Marcus Welser, Isaac Vossius, Claudius Salmasius, Jean Hardouin, and Johann Daniel Schöpflin; it was first printed with the title: "Dicuili Liber de mensura orbis terrae ex duobus codd. mss. bibliothecae imperialis nunc primum in lucem editus a Car. Athan. Walckenaer" (Paris, 1807).
- J. J. Tierney, Dicuil: Liber de Mensura Orbis Terrae, (Dublin 1967).
- Esposito, Mario; Dicuil (1906). "An Unpublished Astronomical Treatise by the Irish Monk Dicuil". Proceedings of the Royal Irish Academy. Section C: Archaeology, Celtic Studies,History, Linguistics, Literature 26: 378–446.
- Hartig, Otto. "Dicuil." The Catholic Encyclopedia. Vol. 4. New York: Robert Appleton Company, 1908. 18 Feb. 2015
- "Dicuil". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
- Chisholm, Hugh, ed. (1911). "Dicuil". Encyclopædia Britannica (11th ed.). Cambridge University Press.