Taxpayers are contributing £5m a year to fund the wages of public sector staff who spend their time on trade union business.
At least 140 full-time equivalent employees work on union duties in Northern Ireland.
The annual cost of covering their wages tops £4.7m - with the five health trusts alone running up a bill of £1.5m.
And the real cost is likely to be even higher because many public sector bodies kept no accurate records of time spent on union business.
David Hoey, co-ordinator for the TaxPayers' Alliance in Northern Ireland, said it was wrong for the public purse to be subsidising unions.
"The size of the public sector in Northern Ireland is far greater than in any other part of the UK, and taxpayers will be surprised that so many staff who should be working for them are in fact working for trade unions," he said.
"While some time is of course justified, the scale and cost of facility time across Northern Ireland - where it is monitored at all - seems to go well beyond what anybody would consider reasonable at a time when we're trying to make necessary savings.
"Trade unions are valuable contributors to our public debate and at their best can be a constructive force for better public services.
"However, taxpayers shouldn't be subsidising them."
The spending relates to facility time, where taxpayer-funded public servants are given time off normal duties to work on union activities. These can include discussing issues such as redundancy and pay or attending activist meetings to organise strikes.
The payments will anger many of the thousands of people affected by a massive public sector strike earlier this year. The TaxPayers' Alliance surveyed all Northern Ireland government departments, public sector organisations and arm's-length bodies. It found Northern Ireland health trusts provided facility time to the value of £1.5m in 2013-14.
The campaign group claimed this could have a knock-on effect, because frontline staff need to be replaced - often by agency or locum staff - resulting in taxpayers paying two people for the same job.
The Northern Ireland Housing Executive afforded facility time equivalent to the value of £350,000. The bill for Translink was more than £175,000, and at NI Water it was over £80,000. Despite many public sector organisations automatically deducting the cost of collection for trade union dues, only three organisations charge a fee to the union for this service.
It means taxpayers are, in addition to facility time, subsidising the union's administration costs.
However, Patrick Mulholland from the Nipsa union defended the costs of facility time.
"This is part of the same union-bashing process that the Tories have engaged in with the anti-trade union bill," he said.
"It is aimed at neutering the unions, weakening their power to fight back against the cuts and austerity agenda. Workers have the right to representation, a right which is enshrined in legislation.
"Therefore they require the services of trade union officials to provide that representation, particularly at a time of cutbacks. The money spent on trade union duties is minuscule compared to each organisation's total budget."