Murphy Brewery and other Irish wholesalers fear a £28.5m deal has given its rival Guinness too much control over their sector.
Guinness has been given the go-ahead to take over one of Ireland's biggest wholesalers, United Beverages Holdings (UBH).
But the Irish High Court rejected the warnings from four other drinks wholesalers, including Murphy Brewery Ireland, that it would give Guinness too much control over the market.
Instead, the judge backed the Competition Authority which originally decided that the takeover did not break anti-monopoly rules.
Murphy Brewery is owned by Heineken and licenses its Murphy's stout in the UK to Whitbread.
Guinness Ireland, part of drinks giant Diageo, already owned 30 per cent of UBH but is acquiring the remainder for £28.5m.
It means the brewer now supplies three-quarters of Ireland's drinks products and is the country's largest distributor of both its own and competitors' products.
To smooth the takeover with the authorities, it sold its 49.5 per cent stake in the Cochrane & Cantrell Group to Allied Domecq for £233m. In January Allied sold the whole business to venture capitalists BC Partners for £519m.