Nobody knows where the word tantrum comes from, though it has been around for at least three hundred years. Some sources say that it originally had the meaning of penis. How it came to mean a fit of temper is unknown.
Daniel Cassidy claims that the word derives from the Irish teintrighim, which is defined as ‘I flash forth’. This is a really bizarre claim and there is absolutely no chance that it is correct. For one thing, tintrím (modern spelling) is not a verb which is given in Ó Dónaill, though it is given as teintrighim in Dinneen. While teintrighim is given as a headword in Dinneen (who tends to give the first person form of verbs), it is hard to see how ‘I flash forth’ would really be used in any imaginable context, except by some Celtic thunder god. There is certainly no evidence of the word teintrighim being used as a noun like tantrum. A tantrum would usually be translated in Irish as a racht feirge or a taghd or a spadhar. There is no evidence even of tintríocht (the abstract noun meaning fieriness) being used to mean tantrum.
As usual in Cassidy’s ridiculous book, it’s complete nonsense.