This piece was previously attributed to Rubira and called "Estudio en Mi" (Study in E). However authorship was widely disputed and a number of other composers have been suggested. Due to the uncertainty the piece is now usually known as "Romance Anonimo" (or Romance by Anonymous) and probably dates from the 19th Century. The piece has been covered and arranged by many different artists including the likes of Julio Iglesias, Mike Oldfield and My Chemical Romance. In terms of interpretation, the first note of each triplet provides the melody and should be sustained for the whole beat.
The image below is the first page of the sheet music, and here you can download the full sheet music for Romance Anonimo for guitar in PDF format.