In Daniel Cassidy’s insane and inane book, How The Irish Invented Slang, Cassidy tried to prove that hundreds of words in English derive from Irish.
His methodology was simple: he hunted through Irish dictionaries to find a word which resembled the target word in English. When he couldn’t find anything suitable (which was usually the case), he took two or three Irish words and combined them into a ‘well-known phrase’ which had never been used in Irish, and for which Cassidy was happy to provide a fake definition.
Occasionally, Cassidy found words which seemed a good fit (at least for some of the meanings) but made no attempt to establish whether they were loanwords into Irish or loanwords from Irish to English.
Cassidy claimed that the word gaff meaning a boat-hook comes from the Irish gaf or geaf. However, gaf or geaf really comes from English and English got the word from Provencal gaf via French. The word gaffe meaning a blunder, is the same word. A quick search on the free and fully-searchable Electronic Dictionary of the Irish Language will confirm that gaf/geaf is not an ancient word in Irish. It is plainly, obviously and clearly a loanword.
Incidentally, the unrelated word gaff meaning a home or a place is from Romani gav.