Sian is Welsh not English.
Séan(pronounced Shan) is the Irish for old and it is also used for wise as well. Hence Shannon and Senan/Séanan. The chief meaning could have been extrapolated from the old and wise meaning - ie an elder.
There is an Irish name surname Shanley. I found this http://www.houseofnames.com/xq/asp.fc/qx/shanley-family-crest.htm
which states it comes from Séan +laoch(hero) which seems plausible.
But I also found this http://www.surnamedb.com
/surname.aspx?name=Shanley which argues for an English meaning. It is not unusual for a surname in Ireland to have both an English/Scottish/Welsh and Irish origin.