The sports retailer is buying the department store chain for just over €100m
The Mandate trade union has said it is seeking an "immediate meeting" with the administrators for the House of Fraser group.
The union, which represents Irish workers in the Dublin outlet, said they are uncertain of their future after the administrator temporarily closed the store on Friday morning.
Sports Direct, owned by Mike Ashley, earlier purchased the House of Fraser department store chain for stg£90m (€100.32m) in cash.
However the deal does not include the Dundrum store in Ireland - which has more than 200 workers, along with concession staff.
Gerry Light, Mandate assistant general-secretary said: "Our members are obviously very concerned about the closing of their store today and about their future.
"We are requesting an immediate meeting with EY to discuss the future of the store and the potential consequences for our members."
In a statement to the London Stock Exchange, Sports Direct said it had acquired 169-year-old department store chain, saving "around 17,000 jobs."
The sports clothing and equipment retailer already owned an 11% stake in the brand.
It said it will buy the business and assets of House of Fraser from the administrators of the company's main operating units - House of Fraser Ltd, House of Fraser (Stores) Ltd and James Beattie Ltd.
Under the deal, Sports Direct acquired the House of Fraser brand, all of its UK stores and all of the stock in the business.
Earlier on Friday, House of Fraser appointed Ernst & Young (EY) as administrators, after it failed to reach a "solvent solution" with its main creditors.
Back in June, 31 of the retailer's 59 stores across the UK were earmarked for potential closure.
However the Dublin outlet at the Dundrum Town Centre was set to survive.
A spokesperson said the Dublin branch was not up for possible closure, as it is held under a separate legal entity.
Dundrum is the company's only store in the Republic of Ireland, alongside a store in Belfast.
The House of Fraser website has been taken down for maintenance works.
In a message, it reads: "We're currently working hard to make some improvements to the website.
"Don't worry, we will be back up and running as soon as possible.
"While we're otherwise occupied, check out our social channels below to see what we've been up to."