Siberian Chiffchaff, Kerries, near Tralee, 6th December 2013 (David O'Connor).
Any Chiffchaff seen in winter is worth a second look, as Siberian Chiffchaffs have been increasingly identified throughout Ireland, though whether this is because they are occurring more frequently or have been previously overlooked is open to debate. They are usually treated as a distinct subspecies of Chiffchaff (Tristris), but have been treated as a full species by some.
Several have been seen in Kerry in late autumn and winter recently, including two individuals together at Ross Castle in January 2012. See photos of those birds in this post HERE and HERE, and one from 2009 HERE (also at Ross Castle), and compare them with the above individual. Although very similar to Chiffchaff, the generally 'cold' tones to the plumage, slight pale wingbar and pale tertials are all indicative of a 'Sibe Chiffer', as well as the different call.