Travellers, have historically been separated from the larger group of Irish society; customs and practices emphasise the division and reinforce Travellers' internal security: the main symbol of their difference, their linguistic code, which operates as a secret or protective language, is known variously as Shelta, Gammon or Cant. The Thesis provides a critical examination of the historical and academic analysis to date of Shelta and reports on the results of an empirical study carried out by the researcher into sociolmguistic aspects of Shelta in Ireland today. Theories about the background and history of Irish Travellers are reviewed and the relationship between Travellers and Gypsies is analysed. Traveller identity and aspects of Traveller culture are examined with particular emphasis on family organization and ritual cleanliness because these show how Travellers maintain the borders between themselves and settled people. The Gypsy language, Romani, is well known and has been widely studied. Shelta has been less documented and its relationship with English Cant has been obscure. A theory is put forward about this historical relationship. Up to now, Shelta has generally been considered to be an artifically devised jargon constructed for disguise purposes. It is suggested, however, that there is reason to believe that Shelta may be the remains of a natural language augmented by a disguised vocabulary from Irish and English, having moved to an English syntactic structure. In recent years, Travellers have undergone rapid social change: their former patterns of employment have been rendered obsolete and there has been a movement towards the towns. Changes in Shelta are examined in this new context and the future role of Shelta as part of Traveller culture is considered.