The Communications Workers' Union says it intends to increase its membership from 20,000 to 23,000 over the next three years by targeting call centre workers and employees in emerging telecommunications firms.
It has started a new recruitment campaign which is overseen by a union sub-committee headed by official Mr Chris Hudson.
He said the union had not been recruiting outside of An Post and Telecom Eireann in the past few years and now had to establish the old principles of union organisation.
"We will be leafleting and talking to workers to try and get their interest," he said. "When we have a few people interested we will set up small organising groups in companies who will try to sell the idea to their colleagues."
He said 100 workers at the US-owned postal group, UPS, had recently become members of the union. He said even in companies which refused to negotiate directly with trade unions, such as Esat Digifone, there were members of his union.