Zelazny wrote that the language spoken in Amber is called "Thari". This likely is a reference to the secret language of the Irish Travellers (i.e. local Irish migrants), known sometimes as "Shelta Thari". The word Shelta first appeared in 1882 in the book 'The Gypsies' by 'gypsiologist' Charles Leland, who claimed to have discovered it as the 'fifth Celtic tongue.' Leland and others since (including George Sampson and Basil Ivan Rakoczi) have asserted that it is of ancient origin. The ancient druids were known to have spoken a secret language -- thus some conjecture was that the secret language of the druids was adopted by other travelling folk to conceal their speech. The mythology of Amber is syncretic fantasy, but often draws on Celtic roots -- thus he chose this as the universal mother tongue.
In reality, it seems that Shelta is a cant based on Irish and English, with a primarily English-based syntax. Shelta's vocabulary is based largely on Irish, with many words inverted in a style not unlike French verlan slang; for example, the word for 'girl' is lackeen, from the Irish cailin, and the word rodas, meaning door, has its roots in the Irish doras.
Within shadow worlds of Amber, then, Shelta Thari is the ancient true language used by the druids and presumably ancients before them. Some mystique may be attached to the Irish Travellers themselves. As background, I give a brief overview.
Irish Travellers (sometimes known as "itinerants" or "Tinkers") are a very small minority group in Ireland. They make up less than 1% of the population with approximately 23,000 people in the Republic and another 1,500 in the North. It is also estimated that there are about 15,000 Irish travellers in Britain and another 7,000 in the USA. Irish Travellers belong to a distinct ethnic group within Ireland. They have their own language, beliefs and social customs which have been made stronger over time due to their exclusion and marginalisation from mainstream "settled" society. Occasionally Irish Travellers have been confused with the Roma or Gypsies in England, who despite centuries of coexistence, cultural interchange and limited intermarriage, remain a distinct people.
Until not so long ago Irish Travellers were referred to as "Tinkers". This word referred to their occupation as tinsmiths and metalworkers and was derived from the Irish word "ceard" (smith) or "tinceard" (tinsmith). This word is now generally used in a derogatory sense. Most of the Travellers' traditional crafts such as spoon-mending, tinsmithing and flower-making have gone by the way now as a result of urbanisation and the introduction of plastic and industrial technology.
Traveller musicians have included the great uilleann piper Felix Doran and the world-renowned folk musicians The Furey Brothers. The music and singing of the Irish Travellers have been in decline since urbanisation and the arrival of television. Fewer Travellers now rely on singing and making music for their livelihood as in times gone by.
There are a number of theories as to the origin of the Irish Travellers. Their secret language, Shelta, and the evidence of various historical references to them would seem to indicate that they are the remnants of an ancient class of wandering poets, joined by those who were pushed off the land during different times of social and economic upheaval such as Cromwell's campaign of slaughter, the Battle of the Boyne (1690) and the Battle of Aughrim (1691). Many of the Travellers may also be the descendants of people who were left homeless as a result of the Irish potato famines of the nineteenth century.