The tinwhistle will play easily in three keys.  Besides the "native" key that plays without half-holing or cross-fingering any notes, a whistle plays in two other keys where only one note needs to be half-holed or cross-fingered.  The chart below illustrates what keys can be easily played on a whistle of any given key.  Note that whistles are not readily available in all keys.  The most readily available whistle keys are D, C, and B-flat.  Inexpensive whistles are also available in the keys of Eb, F, and G.  Expensive custom whistles are available in most keys but you may have to wait months for delivery.