The stack is substantial enough, and old enough in appearance, that it has long been cited as possible evidence of a structure predating 1700, which would make this one of the older surviving buildings in a town already well supplied with medieval remains.

The building, formerly known as The Old Forge, was flagged by Craig and Garner in 1975 as having a notably heavy stack on its southern gable, and this detail was taken up again by Farrelly and FitzPatrick in 1993, who suggested it might point to a pre-1700 origin. The roof pitch is steep, another feature sometimes associated with earlier construction. However, an architectural assessment carried out in 2007 by Fallon found no structural evidence of a pre-1700 building on the site. Instead, the assessment proposed that the chimney stack originally belonged to the adjoining four-bay building immediately to the south, and that it was only later pressed into service for Lonergan's fireplaces through the northern side of the shared party wall. In other words, the feature most often cited as evidence of age may have arrived from next door rather than being original to the pub itself. The building has been modified and extended over the years, which complicates any reading of its fabric, but the chimney remains, doing its work on both sides of the wall regardless of which building it first belonged to.