It has been produced for the Nuance Group, a large travel retail operator with 50+ shops around the world. The malt, I would confidently guess, was made at Bushmills distillery, and has been finished for 8 months in sherry butts. According to Tullamore Dew's Global Brand Ambassador, John Quinn, "the result is a very pleasant combination of maltiness combined with characteristic Tullamore DEW citrus notes and a subtle sherry sweetness."
There was another sherry finish released a year ago, a 12-year old. At the time it was exclusive to travel retail operator, Heinemann, but it is now available at Dublin airport too.
Sherry finishing at Tullamore's own warehouse is one way it can add distinctiveness to whiskey made, for the moment, at competing distilleries. Phoenix's pot still component is matured an extra two years in sherry, while its predecessor, the Black 43, contained pot still matured for one year in sherry. The Black, now discontinued, was very well-received in the Irish whiskey community, while Phoenix was a hit at the most recent Irish Whiskey Society tasting.
I've sampled the Phoenix myself twice in the last week and rate it very highly. One for my own shelf, definitely. I have yet to have the opportunity to acquaint myself with either of the Tullamore Dew sherry finish single malts.