The Mike Ashley-owned business is acquiring minority shareholder Sandra Minor's holding in Warrnambool, on top of its current 50 per cent stake in the business.
The business has also entered into a conditional put and call option agreement with Mark Heaton, Hugh Heaton, John O'Neill, Warrnambool and Katipo Limited over the remaining shares.
In total, Sports Direct will pay €47.5m (£31.1m), which will be funded from the group's operating cashflow and existing bank facilities.
Sports Direct's share price was down 1.5 per cent in early trading.
Heatons has 44 stores in the Republic of Ireland and 10 in Northern Ireland, some of which have Sports Direct concessions. In addition, Warrnambool owns and operates a further five standalone "Sports Direct" branded stores in Northern Ireland.
Ashley has long wanted to increase his empire's reach beyond mainland UK and this will allow Sports Direct to build a stronger presence in Ireland.
For the year ending 30 April, Warrnambool had consolidated revenue of €219.3m, gross assets of €158m, EBITDA of €17.2m and profit before tax of €10m.
Sports Direct chief executive Dave Forsey said: "We look forward to accelerating investment into the existing store portfolio and strengthening the Heatons and Sports Direct brands across Ireland".