According to Wikipedia, Charles Leland referred to the language Shelta (sth) as the fifth Celtic language (family), though with at least Irish Gaelic (gle), Scottish Gaelic (gla), Manx (glv); Breton (bre), Cornish (cor) and Welsh (cym), there are certainly more than five.
The speakers of Shelta are known as Travellers, a people also commonly known by the derogatory term “Tinker” because of the tin work they are known for.
Richard Waters has a Website dedicated to the Travellers in the US, called Travellers’ Rest. This site includes English > US Shelta and US Shelta > English dictionaries as well as links, music, essays and notes about some of the controversies surrounding the Travellers.
Although related to Gaeilge, the syntax is largely based on English.
Parts of R. A. Stewart Macalister’s 1937 The Secret Languages of Ireland can be found at the Internet Archive Wayback Machine and Kobo Books, the vocabulary starting on page 174. Some of the other parts can be found on those sites as well.