What to drink on St Patrick’s Day? It is not a time for green beer, and after a stout or two, you need to move on to the uisce beatha. But what’s the best Irish whiskey? There are single malts and blended whiskeys – both made in the island’s quirky way (Bushmills single malt is triple distilled, for example, instead of the Scottish double distillation). But pot-still is the uniquely Irish style at the heart of the nation’s whiskey culture (Irish blends such as Jameson use pot-still, not malt). So, what exactly is it?

Pot-still is made in the same style of stills as single malt – copper kettles that boil a beer to collect new-make spirit in batches. The difference is the recipe. Where malt whiskey (or whisky in Scotland) is made purely of malted barley, Irish pot-still whiskey starts with a mixed-grain mash bill, with unmalted barley as well as the malt, plus an allowance for other grains such as oats, wheat or rye.

If it sounds similar to American whiskeys such as bourbon or rye, there’s a reason: Irish emigration during the Great Famine in the 19th century, when pot-still Irish whiskey was popular around the world.

Alex Huskinson, one of the spirits experts at The Whisky Exchange, explains the difference in flavour: ‘The three big flavours I consistently get from malted barley in any whiskey are nuts, citrus and honey. Unmalted barley is much brighter and greener – spicy but very fresh spices, like green cardamom; oily but light like linseed oil; fruity, adding pears and green apples on top of peaches’.

All Irish whiskey declined in the 20th century but, as distilleries closed and amalgamated, two pure pot-still whiskeys survived – Redbreast and the Spot whiskeys – Green Spot, Red Spot etc, both produced at Irish Distillers’ Midleton distillery. Although they have slightly different ratios between malted and unmalted barley (Redbreast is roughly 50:50, while the Spots are 60:40 in favour of malted barley), neither uses any other grain.

Unsurprisingly, as Irish Distillers was the most significant pot-still whiskey maker in 2014, when rules governing its production were belatedly drawn up, they reflected the recipes for Redbreast and Spot. The mash bill for a pot-still whiskey must include a minimum of 30 per cent malted barley (never peated); a minimum of 30 per cent unmalted barley; and, notably, a maximum of 5 per cent other grains.

Not long after the rules were set, an Irish whiskey boom saw new distilleries open that would want to break them. Boann in Meath and Blackwater in Waterford are making Irish whiskey with mixed-mash bills featuring oats and rye in higher proportion than the rules allow, so they cannot be labelled “pure pot-still Irish whiskey”. Huskinson says, ‘It means there is even more diversity of flavour. Rye adds warm spices; oats accentuate the nuttiness a little bit, but they do more for texture, adding creaminess.’

Although non-compliant, these are mash bills that have precedents in the 19th century, so there is a strong movement to amend the rules to allow more variety. Irish Distillers are not standing in the way of the amendment, so this should happen fairly soon. In the meantime, they are available, just without the ‘pot still’ label. Here are our current favourites, for a genuinely Irish St Patrick’s Day celebration…