The Dublin TD accused ICTU leaders of "empty posturing" after leading tens of thousands of people in protests over Ireland's debt burden.
"The property tax will be the biggest austerity measure of all, and if fully implemented along with water charges will see bills approaching €1,000 in a few short years," he said.
"ICTU's manoeuvring is to deflect criticism and pressure away from the Government's, and in particular Labour's, imposition of savage austerity measures."
Mr Begg had told RTE's 'This Week' radio show that the property tax was "a democratic decision of parliament, and it is not our business to interfere with that".
Marches in Dublin, Cork, Galway, Sligo, Limerick and Waterford drew big crowds, with ICTU estimating that over 110,000 took part in the weekend demonstrations. Gardai put the figure at a much lower 50,000. But whatever the number, there was no doubting the marchers wanted their voices heard.
In Dublin, the dreaded 'A-word' was the demon of the day, with speaker after speaker lashing out at the Government's austerity measures.
The feelings of most who took part in the demonstration in Dublin was summed up by marcher Jimmy Nealon from Cabinteely.
"This austerity thing is doing my head in. It seems to be all we have talked about for the last few years," he said.
"What this country desperately needs, more than anything else, is jobs. Cutting, cutting, cutting in the interests of austerity is causing untold misery."