Transport company Stobart Ireland, which has been threatened with strike action from Thursday, has claimed that its own ballot of drivers showed that only a minority backed industrial action.
However, SIPTU has accused the company of engaging in coercion and union busting tactics, and stressed that Thursday's stoppage will go ahead.
The strike is expected to cause huge disruption to Stobart's main distribution client, Tesco.
On 15 February, SIPTU announced that Stobart drivers had voted to strike and served notice of industrial action over fatigue issues, inadequate rest periods and rosters.
They will stop work from 6am on Thursday to 6pm on Friday.
However, the company decided to hold its own ballot of drivers yesterday, which was supervised by retired Assistant Garda Commissioner Martin Donnellan.
Stobart Ireland says its ballot showed that only 10% of its drivers favoured the strike.
Of 122 drivers eligible to vote, 16 were absent due to sickness or holidays. Of the remaining 106, 58 cast a ballot.
They say 45 drivers opposed strike action - and that only 12 drivers - or 10% of the total workforce - voted in favour. One vote was spoilt.
Stobart says the 45 who opposed strike action represented 78% of those who voted and 37% of all affected staff.
The company described the vote as "independent and transparent" and urged SIPTU to call off the strike.
However, SIPTU Official Karan O'Loughlin accused Stobart of intimidating and coercing drivers to participate in the "hastily arranged ballot".
The union pointed out that over half the relevant workforce had not taken part in the ballot - and that those opposing industrial action represented only 37% of the total number of drivers.
SIPTU has complained to the Health and Safety Authority about Stobart Ireland phoning workers to instruct them to return to work to vote after they had finished their shifts - claiming this breaches their legal entitlement to uninterrupted rest periods.
It also urged Stobart's biggest customer to break its silence on the dispute.
