The Irish supermarket sweeps are as tough to call as the Grand National – one week’s hot favourite is the next week’s non-runner, as Tesco’s most senior executives will be all too aware this week.

Five years ago the British retailing giant’s growth in the Republic appeared to be unstoppable as it opened stores of all shapes and sizes and watched its market share grow to nearly 30 per cent.

Then, almost overnight, things shifted.

While Tesco is by no means circling the plughole in some class of retailing death spiral – it remains the largest supermarket chain in the State – it has watched its market share fall every month for nearly two years and appears helpless when it comes to reversing the decline.

The problems it is experiencing in the Republic have been mirrored in the UK and its chief executive Phil Clarke announced his intention to step aside. In both states, its problem is the same and it comes with a German accent.

In 1998 Lidl came to Ireland and was joined by Aldi a year later. Growth for the discounters was slow in the early days, with Irish people reluctant to swap the branded products they loved for unfamiliar labels and suppliers unwilling to make products to populate the store's relatively spartan shelves.

Then the bubble burst and everything changed. With less disposable income, people became more value-conscious at about the same time that Irish suppliers realised that Aldi and Lidl were good companies to deal with.

Over the past three years, the German discounters have captured almost 4 per cent from the competition and have increased their sales by nearly 40 per cent in an overall grocery market that has grown by just 1 per cent.

Aldi now commands 8.1 per cent of the total grocery spend, with Lidl marginally ahead. According to the most recent figures from retail analysts Kantar, Aldi and Lidl "continue to deliver outstanding growth" and increased sales by 19.5 per cent and 12 per cent respectively in the last quarter. Tesco has a share of 26.3 per cent but its sales but in the first quarter of 2014, it reported declining sales of 5.5 per cent.

It is not just cheap products which have seen both discounters grow so strongly in recent years. The discount sector has changed fundamentally since the Germans came to town at the tail end of the last century. Lidl is selling Pomerol wine for more than €30 a pop, while Aldi’s Specially Selected range of food has been taking on the big boys of the premium sector at international competitions and winning.

Irish shoppers now know that shopping in the discounters does not mean sacrificing quality. Lidl and Aldi now command more than 17 per cent share of Republic’s multibillion euro grocery market

SuperValu, meanwhile, has played a canny game in recent years. Its acquisition of the troubled Superquinn two years ago – for a fraction of the €450 million it changed hands for five years before that – saw it increase its market share by more than 5 per cent at the stroke of a pen.

Its shift towards own-brand goods – with more than 1,000 product lines now carrying its name – has allowed it to cut prices and capitalise on consumers’ realisation that branded doesn’t always mean better.

A look at the numbers, though, shows that while SuperValu has developed its market share over the past two years, the growth has been modest, as has the performance of Dunnes Stores. It is holding its own but, just like Tesco, it will be looking anxiously over its shoulders nervously at the rise of the discounters.