According to Cassidy, the slang term shindig comes from the Irish expression seinnt-theach, meaning a house of music. Seinnt is a common variant of seinm, which means to play a musical instrument, and teach does mean house but the expression seinnt-teach is complete fabrication. It is not attested in Irish and and there are a lot of familiar phrases for a house where music is played and people gather for entertainment, such as teach céilí, teach airneáil, teach airneáin. Teach ceoil (house of music) would also sound reasonable and any Irish speaker would know what you meant. But seinnt-teach (you wouldn’t aspirate the teach, as Cassidy does, so his version of seinnt-theach is a misspelling anyway) is not a real word and it sounds very odd, as if the house is an instrument and someone is blowing into it or hitting it. If you know Spanish, the phrase casa de tocar would give you some indication of why it is odd and improbable.
On some forums, people have defended Cassidy by claiming that Irish speakers suddenly started saying things in a completely different way when they got to the multi-ethnic ghettoes of the New York. They apparently forgot all their grammar and forgot the expressions they had grown up with and were reduced to the level of incoherent grunting imbeciles and conveniently, their grunting imbecilities just happen to coincide with the mad conjectures of a certain Daniel Cassidy. Nobody with half a brain would accept this nonsense. The fact is, if there is no evidence for the existence of something but Daniel Cassidy’s word for it, then it should be treated as not existing. People who regard this as academics trying to maintain a closed shop by refusing to accept the contributions of amateurs are just being stupid. It’s like someone refusing to play pool and simply smashing the pool table with the end of the cue and whacking the balls around the room, then arguing when they are disqualified that the pool establishment has it in for them and refuses to accept their unique and iconoclastic way of playing!
As for the real explanation, the dictionaries suggest that shindig is linked to an obsolete word shindy which means a ruckus. This may be correct but the word shindig also has a certain aptness, in that drunken people dancing clumsily tend to dig each other’s shins. Whatever the explanation, deriving it from an Irish phrase only works if there is an Irish phrase, and in this case there isn’t, because seinnt-theach was made up by Daniel Cassidy.