She will be playing Irene Adler, a character that originally appeared in Conan Doyle's story A Scandal in Bohemia. If I recall correctly she has the distinction of managing to best Holmes. The first line of the story reads,
To Sherlock Holmes she is always THE woman.
I look forward to seeing how she is used in the new version.
Source: http://www.denofgeek.com/television/1002838/sherlock_series_2_casting_news.html