This answer contains spoilers… (view spoiler)[It's in the book. "Lexi" escaped to Dublin after her engagement and ran across an acquaintance of Cassie's when she was undercover as Alexandra Madison. When "Lexi" realized this person thought she was someone else entirely, she went with it and pretended to be Alexandra. That's why the friend said Alexandra had picked up a heavy American accent when she was staying with family (Cassie/Frank's story to easily get Alexandra out when they were done with their undercover work). "Lexi" then picked up the identity and built the life for herself we find in the story. (hide spoiler)]
Mommooshka Thanks, Rebecca, for your great explanation - I had forgotten about that meeting! I found there were so many subtle, complex events throughout that it was one of those books where I went back after I finished it and reread some sections. Although I may have missed or been unclear about a few facts while reading it, they all came together really well at the end which to me is the sign of a well written, very interesting book.