SIPTU's president Mr Des Geraghty has rejected suggestions by Ryanair chief executive Mr Michael O'Leary that he used the non-union airline several times in the past year.
Mr Geraghty has demanded an immediate retraction of Mr O'Leary's "misrepresentative claim" and says he will seek an apology in the courts if necessary.
In the course of a general attack on SIPTU on RTE's Morning Ireland programme yesterday, Mr O'Leary said it was "breathtaking" that "Des Geraghty, or D Geraghty, has flown six times with Ryanair in the past six months".
Mr Geraghty said later he had never travelled on a Ryanair flight "before, during or since" the unsuccessful strike for union recognition by SIPTU baggage handlers in 1998.
He called on Mr O'Leary to apologise and said the Ryanair chief executive was "well aware the person in question is not me".
Mr O'Leary resumed the attack on the union chief yesterday afternoon. If Mr Geraghty was not flying on Ryanair, were the hard-earned dues of union members "being frittered away on higher fares for SIPTU bigwigs", he asked. Did Mr Geraghty get "a special deal" on Aer Lingus and "travel with the other fat cats in business class"?
Responding, Mr Geraghty said: "I don't travel with fat cats, skinny cats or any other cats. Actually, I usually take my car and the ferry when I go on holiday."
"I would normally travel economy class and I travel with Aer Lingus because that's where our members work."
Meanwhile, two tyres burst yesterday morning on a Ryanair 737 from Dublin after it landed at Manchester. All 33 passengers disembarked safely.
According to a spokeswman for Manchester Airport the incident caused delays of up to 45 minutes to other flights yesterday morning.