There's no way irony isn't characteristic of Americans - but I suggest that the difference is that Americans tend to use direct un-ironic speech as the default mode of communication, indicating when they are switching to ironic ("smile when you say that"); the English (and the Irish) tend to use irony as default, especially in situations where some kind of comment is felt to be needed, and are more likely to feel it necessary to give a specific indication when they actually mean what they say ("and I mean that").