O'Reilly's Stout from Pottstown's Sly Fox is one of my favorites in the dry stout genre — an admirably fresh local alternative to Guinness come St. Paddy's Day. It's fascinating to see how it changes subtly, though, with a little help from its Irish whiskey friends for the limited edition O'Reilly's Extra. Jameson, in an initiative to promote its more premium whiskey, the longer-aged "Caskmates," has offered its used barrels to select breweries around the world to age their beers in. Maturing beer in whiskey barrels has become a big national trend, but one that can easily slide out of whack, with beers that too often finish harsh and boozy. But Sly Fox's precision-minded brewmaster Brian O'Reilly was particularly careful in this regard, aging the beer for just two weeks in the casks, which, before the Irish whiskey, once aged bourbon. As a result, this dry stout is still full of its classic dark coffee and roasty notes — emphasized more by the brisker carbonation of CO2 (versus the usually creamier pour on nitro) — but gains an extra basement level's worth of depth from the vanilla and caramel notes of the barrel. At 6 percent alcohol, about two-and-a-half points stronger than standard O'Reilly's, it packs a little more heft that I found ideal for the nippy weather. But the beer hits belly-warming overdrive when sipped by the preferred method of service at the events around town celebrating this duo – with a side shot of the special Jameson for a Caskmates-style City Wide Special. Grab a sip while you can, as long as the "Extra" is just that. It won't last long.

– Craig LaBan

Sly Fox O'Reilly's Extra Stout events: Friday at Good Dog Bar, 224 S. 15th St., $6 a pint, $12 as a City Wide Special with a shot of Jameson's Caskmates;  Wednesday at Butcher Bar, 2034 Chestnut St.; butcherbarphilly.com; Nov. 20 at Varga Bar 941 Spruce St.; vargabar.com; Nov. 22 at Bar 1309 Sansom St.; barphiladelphia.com