The strike, which began at midnight, affected the company's meat and dairy processing plants in Co Cork, as well as a number of its 39 co-op stores.
The union last night confirmed it had placed up to 100 pickets on sites across Munster and in Dublin as a dispute over new work practices escalated.
SIPTU, representing most of the 2,500 workers in the €1 billion a year turnover co-op, reported a determined and buoyant mood on the picket lines.
These were placed on the co-op's milk and pigmeat processing plants and on retail, distribution and other outlets.
There were 17 pickets alone at various Dairygold sites in Mitchelstown, where there is growing concern of possible further job losses in the co-op.
Mitchelstown SIPTU branch secretary Anne Egar said the union had little choice but to take industrial action because the company had unilaterally introduced changes to work practices in breach of its industrial relations agreement.
Ms Egar said the 24-hour strike was extremely effective. All production ceased at Dairygold plants.
The union remained available for negotiations at all times but the possibility of further industrial action was not being ruled out, Ms Egar said. A spokesman for Dairygold said the co-op was able to proceed with some work despite the strike. Services to retail customers were managed well and a significant number of stores were fully or partially open with management staff on duty.
Liquid milk deliveries were made to retailers but a door-to-door service by CMP in Cork was suspended for the day. Hauliers were also out collecting milk and delivering bulk feed and fertiliser as normal.
The spokesman said the company could not understand why workers decided to embark on strike action when there is a Labour Court hearing scheduled for Cork tomorrow.
"Rather than engage in destructive strike action, SIPTU should consider constructively working with Dairygold to save the business and restore it to profitability and growth," he said. Dairygold which suffered 500 redundancies before Christmas has denied breaching its labour relations agreement.
Following a sizeable drop in profits in 2002, Dairygold is currently in the middle of a major restructuring and cost reduction programme.