From "Taprobana", the Greek name for Sri Lanka, itself derived from the Sansrkit tamraparni meaning "copper-leaved."
In Iolo Morgannwg's mythology, the so-called "Summerlands" (Gwlad y Haf)1 that was the original home of the Britons before being lead westward by Hu Gadarn.
The number of the saints of Sicomorialis,
And isle of Deffrobani.
And the holy multitude that blessed
Water, wine, hostile men destroyed.
Book of Taliesin. II (13th century)
There are three pillars of the Nation of the Isle of Britain. The first was Hu the Mighty, who brought the nation of the Cambrians first to the Isle of Britain; and they came from the Summer Country, which is also called Defrobani (that is, where Constantinople now stands); and they came over the Hazy Sea to the Isle of Britain where they settled...
The Iolo Triads (18th century)
In the Welsh Charlemange cycle, evidenced in the Red Book of Hergest and White Book of Rhydderch, there is a romance titled "The Expedition of Charlemagne to Jerusalem and Constantinople, and his adventures with Hu Gadarn." This is a translation of the twelfth-century romance "Le Pèlerinage de Charlemagne". In this humorous tale, Charlemagne and his retinue, while on crusade in Jerusalem, spend time in Constantinople and proceeds to best the Emperor Hugo after Charlemagne's wife unfavorably compares him to the Eastern king. It was translated into Welsh in the thirteenth century, probably at the behest of Maredudd ap Rhys.
There is not a single mention of Defrobani in the text.
In the mythology of Iolo Morgannwng, however, Defrobani is not only the original homeland of the Cymry, but is the land ruled by Hu Gadarn, who then led the Cymry to Britain. However, in order to maintain that Hu Gadarn was the ruler, he decided that Defrobani refered to Constantinople, not Sri Lanka. He never explains this change in location or why Constantinople is never once called Defrobani in the Welsh manuscripts.
As stated above, Defrobani is the Welsh approximation of the Greek Taprobana, now known as Sri Lanka or Ceylon. It is first mentioned in the West by Ptolemy, but really enters the Western consciousness in the writings of Marco Polo and John Mandeville. Polo describes Taprobana as having a Mount Serendib, upon which is buried some artifacts belonging to the Biblical Adam; it is also then identified with Eden.
Knowing this, it is not strange that Iolo would identify Defrobani with the original home of the Britains. So as Sri Lanka/Defrobani was identified with the original home of mankind, and Hu Gadarn was identified with Constantinople, Iolo decided to move the association with Sri Lanka away from that to Constantinople, and make that the original home of the Cymru. Or something. It's all very confusing, not the least because Morgannwg was an opium addict whose writings repeat and at times even contradict themselves.
What is not fully understood, however, is why he picked an obscure figure from a French romance about Charlemage and made him one of the founders of Britain. Aside from Morgannwg's heavy opiate use, there may be another reason:
In the medieval Welsh romance "Peredur" we have an "Empress of Constantinople" who makes the title character her lover for fourteen years. Constantinople (or really the Byzantine Empire) was often incorporated into Arthurian legend, and so it was also incorporated into this Charlemagne romance. The comparison with Peredur is significant, as both Hu Gadarn and Peredur are said to have drawn an afanc out of a lake, in Peredur's case with the help of a stone that rendered him invisible. The stone was given to him by the Empress of Constantinople. It is likely that Iolo borrowed elements of the Peredur story, perhaps remembering the story of Charlemagne and Hu Gadarn in Constantinople. Or, of course, it could have just been his drug-addled brain unable make any sense of its own muddledness.
Defrobani has also been rationalized as Dyffynbanu2 or Dyffynalbanu, supposedly refering to "the deep valleys of Albania." This interpretation is largely dependant on the medieval idea that the Celts were originally Scythian. Though there was certainly trade and cultural contact with the Scythians, the Celts were decidedly a different culture, speaking a different language group. However, the identification with the Scythians, and the Scythians with the Lost Tribes of Israel is still an unfortunately-present idea, being part of the "British-Israelite" nonsense.
1. Gwlad y Haf: this is actually the name for Somerset, derived from the Old English Somersæte, meaning "land of summer", maybe because the climate of southeastern Britain is much milder than the rest of the island. The region itself did not fall to the Saxons until at latest 710 CE, after the Battle of Llongborth. Prior to that time, it was mostly part of Dumnonia.
2. This is popular among some British-Israelism proponants. I found this particular identification in The Religion of Ancient Britain, Historically Considered by George Smith (1846)