Well, Ireland gets a lot of UK TV so an English accent is not that big a deal. But the American ones make you take more notice - if u know what I mean. There are also more than a few radio adds using real and fake American accents too for concerts etc. Back to TV, fast food and movies tend to use American as well. (The latter goes for both Australia and here eg: Domino's Pizza often uses an American VO).

That Ingham one most likely used English to let ppl know that even though it's frozen chicken pieces it's good quality food (hah!) b/c Australia was a convict/colony nation and the upper crust were British, y'know. pip. pip. ;p This also works with truly ocker accents as well, in Australia. If they want you to identify as something true blue Aussie, they use a broad and stereotypical type accent. "Onya mate" type stuff. example would be beer ads. It's interesting stuff, actually. A lot of psychology goes into commercials, as I'm sure you know :)

But yes, generally speaking in Australia an American accent is cool. And English would be cool to the snobbier types ;p In Ireland, American accent is used slightly differently. I'd say it's to give the product more of an authentic feel 'Direct from America' type thing. It makes an impression :)