Update 6.15pm: Minister for Transport, Shane Ross, has reiterated that the Government cannot intervene in the Dublin Bus dispute as it could be seen as "a commitment to open the State chequebook".
The Minister said his department "greatly regrets the grave inconvenience" on the travelling public in Dublin due to the dispute.
Mr Ross said he is "acutely aware" of calls for him to directly intervene but must reiterate, that "as any Ministerial intervention could be interpreted as a commitment to open the State chequebook, it would be inappropriate" for him to do so.
He again called on management at Dublin Bus and the unions to engage with each other immediately.
Update 5pm: Dermot O'Leary, general secretary of the National Bus and Rail Union (NBRU), has blamed political leaders and transport chiefs for not negotiating with bus drivers on what he described as a long overdue pay rise.
"What is particularly galling here is the undisputed fact that this dispute will ultimately be settled around the negotiating table," he said.
"Allowing service disruptions to become the de-facto norm in the nation's capital is a sad indictment on those who are both elected and appointed to provide this vital service."
Trade union Siptu said five unions, including itself, the NBRU and Unite, had agreed to ratchet up the industrial action at a meeting on Thursday.
Organiser Owen Reidy claimed Dublin Bus bosses and Department of Transport officials have little interest in resolving the row.
"What is needed is for all sides to commit to a serious negotiation process and display fresh thinking concerning the funding of Dublin's public bus system," he said.
"Workers are no longer prepared to be a soft touch whose pay is suppressed to subsidise a declining state subvention."
Unite has already threatened a potential further escalation when the unions meet again next month to review their action.
Dublin Bus has denounced the walk-outs as unnecessary and unjustified.
"To date, this industrial action has cost the company in excess of four million euro and continues to impact the financial stability of the company," a company spokeswoman said.
"We will now assess the full implications of today's announcement."
Update 4pm: Siptu has served notice of further strike action on Dublin Bus during September and October.
Siptu and the other four unions agreed a series of dates at a meeting this morning.
The stoppages will take place on Tuesday 27th and Wednesday 28th, September. These are in addition to the 48-hour work stoppage already scheduled for next week - Friday 23rd and Saturday 24th September.
In October, they will take strike action on Saturday 1st, Wednesday 5th, Friday 7th, Monday 10th, Wednesday 12th, Friday 14th, Tuesday 18th, Wednesday 19th, Monday 24th, Wednesday 26th and Saturday 29th.
Siptu's Owen Reidy, said: “Despite the fact that we are currently in the third day of strike action resulting from this dispute, it would seem that the management of Dublin Bus and the Department of Transport have little interest in resolving the outstanding issues.
“Our members are disappointed that the only response so far from the CEO of Dublin Bus to this dispute has been to call for talks at the Workplace Relations Commission to discuss a Labour Court recommendation that has already been rejected by over 90% of our members. It is not a genuine attempt to find an agreed resolution to this dispute.
“The trade unions at Dublin Bus have a responsibility to resolve this dispute. However, this cannot be done without input from management supported by the Department of Transport. What is needed is for all sides to commit to a serious negotiation process and display fresh thinking concerning the funding of Dublin’s public bus system. Workers are no longer prepared to be a soft touch whose pay is suppressed to subsidise a declining state subvention.”
Siptu's John Murphy, said: “It is not acceptable to ask workers to comply with three comprehensive restructurings of Dublin Bus, which have resulted in the company returning to profitability, for little appreciable reward. What is even less acceptable is that rather than some of these profits being redistributed to a workforce that has not had a pay rise in eight years the National Transport Authority simply takes €2 million from revenues.
“Our members hope that their decision to escalate industrial action will focus the minds of those who have a responsibility to resolve this crisis.”
Update 1.50pm: Dublin Bus drivers are planning to ratchet up strike action with 13 more stoppages planned over the coming weeks.
Union sources have said there will be two more industrial action days in September and 11 in October.
The two strike days in September are additional to the already announced stoppages for September 23 and 24, it is understood.
Update 12.10pm: Unions representing drivers at Dublin Bus are meeting to discuss a possible all-out strike.
They say the industrial dispute will escalate, but that an all-out strike is the "nuclear option".
A 48-hour work stoppage is underway, with another two-day strike planned for next week.
Siptu's Owen Reidy, says it could move to an all-out strike if management at Dublin Bus don't start negotiating.
He said: "It is a possibility, I mean it's the nuclear option, I don't think anybody wants to do it but you take industrial action as the last cause, you do it to try and affect something from the employer to get them to come to the table.
"Quite frankly, if we can get them to the table there is an onus on us to be flexible too, and we will be flexible.
"There's no table to be got to, noone seems to want to talk."
Earlier: 400,000 commuters are making alternative arrangements this morning as the second 48-hour Dublin Bus strike begins.
The drivers are taking the industrial action in pursuit of a of 15% pay increase. The company has offered an 8.25% rise.
Dublin Bus has apologised to customers and urged employees not to strike.
Paul Morton of Mortons Coaches has been in the bus business for 20 years and said the public deserves a better service.
"People are amazing…They weren't grumpy the last two strike days - they had holidays, they took days off," he said.
But he added people should be provided with replacement buses, especially if they have pre-paid for bus travel by way of an annual or monthly bus pass.
"All the NTA have to do for people who have paid for their travel on a monthly or yearly ticket is contact operators, put on a couple of buses on each route, people get on, sit down and go to town.
"They've paid for their ticket, so the NTA should supply a bus for them to sit on."
[The National Transport Authority (NTA) has a 'public service obligation contract' with Dublin Bus for the provision of bus passenger services in Dublin.]
Meanwhile, Retail Ireland is calling on unions to call off the strike, saying it is having a massive impact on trade.
Director of the body Thomas Burke said: "Retailers have been working hard to attract people back into the city centre. This (strike) is really unhelpful. What we don’t need now is prolonged industrial action running into the Christmas period."
AAA-PBP TD Mick Barry said the workers needed to be supported having had no pay increase for eight years despite returning the company to profitability - and that there is significant public support for them.
He called on Transport Minister Shane Ross to intervene, saying it was "a real disgrace the Minister is not instructing Dublin Bus management to engage in meaningful talks".