The tradition died out when new rules forced wine producers to bottle before export. The supply of casks dried up and the wine importers exited the business of maturing whiskey themselves.
The Celtic Whiskey Shop, however, is reviving the practice. The shop also stocks wine and has been asking its various suppliers to send an empty cask or two to Ireland. Cooley then fills the casks with whiskey for "finishing". This is where a whiskey that has been aged for, say, 8 years in an ex-bourbon barrel is refilled into a different cask for the last year or two of its maturation. This can make a very striking difference to the final whiskey.
The shop released the Tyrconnell Ànima Negra, finished in a Mallorcan wine cask, last year. This month sees the unveiling of the Connemara Cask 155 Emilio Hidalgo Amontillado Finish. There is a good story behind it. Bodegas Emilio Hidalgo told the shop that they don't buy or sell casks and, indeed, hadn't bought a new cask in 40 years. That's because they use the solera process for maturing sherry, which never actually empties a cask but draws off a portion and immediately refills it with slightly younger stock.
You can imagine that the sherry over the decades has had time to thoroughly permeate the wood. Contrast this with the sherry casks typically used in an Irish distillery which have been "seasoned" with sherry for only two years. Those have typically been filled with Oloroso sherry. Amontillado is not dissimilar but it has a slightly less oxidised character.
Anyway, they convinced the producer to part with a cask and it has spent the last 12 months maturing some of Cooley's peated malt. It has been bottled at 46% and goes for €110 but is currently on offer at €95. I haven't tried it yet but those who have rave about it. The shop has some tasting notes:
Intense and pronounced on the nose. Aromas reminiscent of kippers smoking, bonfires on the beach, tarred rope, flint smoke, hessian sacks, dried figs, dates, Christmas cake and chocolate. On the palate it starts off sweet and fruity with flavours of sultanas, dates, and candied orange peel then the smoke and spice comes through with notes of peat fires, spiced fruits and sea spray.