The group, which is also behind Rubicon and Tizer, said over 90 per cent of its brands will contain less than 5g of total sugar per 100ml by the autumn of this year.
AG Barr is grappling with a shift in consumer tastes towards low-sugar drinks and is preparing for the implementation of a sugar tax in 2018.
The proposed levy, due to be introduced in April 2018, is aimed at tackling soaring obesity rates.
The industry tax relates to the sugar content of drinks, with a higher amount charged for the most sugary beverages.
Chief executive Roger White said: "Evidence shows that consumers want to reduce their sugar intake while still enjoying great tasting drinks.
"We've responded by significantly reducing sugar across our portfolio in recent years, through reformulation and innovation.
"Today's announcement builds on this progress and we are now expanding our successful sugar reduction plans to include our iconic Irn-Bru brand."
Mr White said the drink, popular in Scotland, will retain its "unique great taste", just with less sugar.
In February, the Cumbernauld-based firm said it is on track to meet full-year profit guidance, but flagged another challenging year ahead.
Last year, AG Barr announced it was cutting 90 jobs as part of a company-wide revamp expected to cost around £4 million.
