I ARRIVED IN IRELAND in late 1994 and that was the first time I heard the term "gobsmacked". A learned Irish flatmate told me "gob" was Irish for "beak" and then the term made sense to me. And right away "gobshite" had a deeper meaning too. If you search Google News for references to the non-hyphenated word, it looks like the first time it surfaced was in Chemistry World on 11 March 1985.
... was just absolutely gobsmacked when the results came in I personally did the analysis and it was just blindingly obvious that every single character ...
Google Books (includes magazines) offers up the New Scientist, March 7, 1985.
Our online dictionaries in Tipperary Institute give the etymology.
gobsmacked, adj. UK slang
Etymology: < gob n.2 + smacked adj. at smack
Flabbergasted, astounded; speechless or incoherent with amazement.
1985 Guardian 23 Feb. 19/3 Last Tuesday I shook him [sc. Sir Stanley Matthews] by the hand.‥ On the red shale Spion Kop where, 40 years ago, I used to stand to see Wednesday play, I would have been described as ‘gob-smacked’.
1987 J. Curran K2 (1989) i. vii. 88 The event that really caused raised eyebrows was that the Frenchman, Benoît Chamoux, had climbed the Abruzzi in only twenty-four hours! ‘Gobsmacked’ was the only word to describe our reactions.
1989 Jackie 25 Nov. 43/1 Won't they be gobsmacked when you tell them that you wrote to me?
I've been out of the States to notice whether anything would gobsmack me at home.
Gobsmacked on Flickr. Image is the gobsmacked Shola.
Bonus Info on Gobbledygook.