"like Virgil’s Silenus, who, his face smeared with the juice of berries, discoursed of Olympian Gods to the innocent shepherds of Sicily"
Drunken Silenus by Rubens
Public DomainDrunken Silenus by Rubens

A woodland creature in Greek mythology, Silenus was part man and part goat. A follower of Dionysus, god of wine and ecstasy, he spent most of his time gorging himself on food and wine but was mistakenly considered something of a sage.

The episode Conrad refers to comes from the Eclogues by the poet Virgil. Discovered asleep in his cave by the shepherds Chromis and Mnasyllus, Silenus is tied up and asked to sing to them. They get more than they bargain for with Silenus’ colourful tales. 

Within a cave/ Young Chromis and Mnasyllos chanced to see/ Silenus sleeping, flushed, as was his wont,/ With wine of yesterday. Not far aloof,/ Slipped from his head, the garlands lay, and there/ By its worn handle hung a ponderous cup. Approaching- for the old man many a time/ Had balked them both of a long hoped-for song-/ Garlands to fetters turned, they bind him fast.