leeneia, your MIL's "Get out!" sounds very much like the context of "Shut up!" Yes, it was used in the context of incredulity and I can see it would get irritating very quickly.

I remember an Irish TV series from several years ago, 'Father Ted' (I just looked it up, it was in the late 1990s) in which the housekeeper kept repeating "ah, go on, go on, go on, go on......." to the point of irritation.

Speaking of irritation, a colleague in my last job used the expression "the irrits" meaning something which annoyed her - as in "it gives me the irrits". Or even "a strong dose of the irrits", if it was even more so. Far from bugging me I quite like it, and sometimes use it myself. She was from Victoria, Oz, so perhaps it was a Victorian thing; they are quite different to those of us who hail from New South Wales with different patterns of speech. My Posh Melbourne Grandmother's way of speaking was very Melbournian despite having grown up and married in N.S.W.; my grandparents moved to Melbourne, Victoria, upon marriage.