According to Daniel Cassidy, author of the idiotic How The Irish Invented Slang, the word hackney, meaning a cab, is derived from the Scottish Gaelic each ceannaich, meaning a post horse (that is, a horse kept at an inn to be lent to travellers or to be used by someone carrying the post whose own horse was too tired to continue).
The Wikipedia article on the hackney cab has this to say:
“The name ‘hackney’ was once thought to be an anglicized derivative of French haquenée—a horse of medium size recommended for lady riders; however, current opinion is that it is derived from the village name Hackney (now part of London). The place-name, through its fame for its horses and horse-drawn carriages, is also the root of the Spanish word jaca, a term used for a small breed of horse and the Sardinian achetta horse. The first documented ‘hackney coach’—the forerunner of the more generic ‘hackney carriage’—operated in London in 1621.”
The term each ceannaich does exist (Cassidy found it in Dwelly’s Dictionary) but it doesn’t sound much like hackney (it would be pronounced akh khyanneekh, with the kh being like the ch in loch) and it doesn’t refer to a carriage. And anyway, Cassidy offers no explanation as to how or why a Scottish Gaelic term would come to be used in the neighbourhood of London in the early decades of the 17th century. From London, it is still easier and quicker to get to France than to the Scottish Highlands. In the 17th century, the road to the Highlands was relatively untrodden and it was culturally and linguistically a very foreign place, even to Lowland Scots.
Incidentally, fans of the Irish-language soap opera Ros na Rún will be familiar with the term hacnaí (the Irish spelling of the English word hackney) for a taxi. The official Irish word for a taxi is tacsaí, but in the soap opera and in the Gaeltacht it’s set in, the word used is always the borrowed word hacnaí.
Cassidy’s claim that hackney and hack are of Gaelic origin is obviously nonsense, like nearly all of Cassidy’s claims and in any case, how could this Scottish and London connection possibly have anything to do with Irish influence on American slang?