Would be pretty unthinkable in
Ireland, since "mac" literally means "son" in Irish. And Mackenna is a pretty common surname. If you were talking Irish, it would only be a man's surname. For example, is a guy was called
Sean Mackenna in English, he would be Seán Mac Cionnaith in Irish.
He might actually be called that if he moved to the Gaeltacht (parts of
Ireland where people speak Irish as their first language.) And if a girl was called
Aisling Mackenna in English, she would be
Aisling Nic Cionnaith in Irish. "Mac" and "O" are only ever masculine surname prefixes. The feminine are "Ní" and "
Nic". So, in Irish it would really make no sense to call a girl Mackenna or
Mckenna.