Wildcat strikes by Bus Eireann workers could happen again, according to the NBRU.
The National Bus and Rail Union insist it wasn't involved in yesterday's action which shut down Irish Rail and Dublin Bus too.
The secondary strikes were unsanctioned, with many held at depots which also facilitate Dart and Dublin bus services. Workers refused to cross the pickets, meaning those services were also affected.
General Secretary of the NBRU Dermot O'Leary says until a resolution is reached, he can't guarantee there isn't more of the same on the way.
"My union wasn't involved and I had to knowledge of it," Mr O'Leary said. "I've been saying for weeks and weeks this could turn into something beyond our control. People will be well able to deduce if this could happen once it could happen again. I would appeal to people at Bus Eireann to remain in discussion with their employers until this issue is resolved."
Both the union and opposition political parties have been calling on the Transport Minister Shane Ross to get involved. Fianna Fáil's Dara Calleary said today refusing to do so wasn't good enough.
"You can't have the whole transport system of the country come to a halt and say it's not up to me," he said. "That's just un-tenable."
The strike is now in its ninth day and centres on a row over pay cuts proposed by Bus Eireann, which says they are necessary to avoid bankruptcy.