Etymologies that make 'perfect sense' aren't always correct. "That is good" just doesn't seem nearly emphatic enough to match how an English speaker would use "smashing." I'm only a learner, but I'd think if you were going English-to-Irish "smashing" would have to be at least an-mhaith, or iontach-mhaith, or something like that.
sean.mullen, yes I am familiar with many of the theories of the etemoligical origans of “smashing”. And theories they are. In the end we simply don’t know. One may prefer one theory over another, and all are entitled. I myself am not promoting any particular theory. I was just stating what the BBC NI Blas the program Giota Beag had broadcast that it originated from is maith sin and that it didn’t seem that far fetched to me. The theory of “smash hit” as it’s origan is less convincing to me. But I don’t discount it because I don’t know and I don’t think that anyone does.
John481518: Are you familiar with the term 'smash hit'? It's that sense of 'smash' that most likely led to 'smashing,' and 'smash' is a blend of 'smack + mash,' which are Germanic words and have no relation to Irish. Folk etymologies are so common because people want there to be connections between similar-sounding words, but the reality is often quite different. 'Cop' (police officer) is another one: it does come from 'copper', but it has nothing to do with the metal in the buttons on their uniforms; rather, a copper is one who cops, or seizes/grabs criminals. That's it.
I think you might be barking up the wrong tree - maybe you need a comparison between "enough", "plenty", "a lot" and "sufficient" (and you could throw "much" and "many" into the mix for good measure).
Some of these uses are idiomatic, and some of them overlap. Similar idiomatic patterns occur in Irish, but they don't always overlap in the same way in Irish and in English.