Tomorrow evening, the serious business begins at cabinet. Ministers have been tipping away at the budget for weeks. But now the real decisions have to be made. There are just three weeks to go before the government has to deliver its four-year plan to Brussels and another three to Budget day.

As ministers sit into their seats around the table at government buildings tomorrow for the first of what is likely to be dozen or more such gatherings between now and 7 December, they will be acutely aware that they are facing into what Alex Ferguson famously described as squeaky bum time.

Nerves are going to become seriously frayed. It's inevitable, given they are looking for €5bn in savings for 2011 and another €10bn over the following three years.

Last December's pre-budget discussions managed to avoid the disagreements of the year before with the cabinet, even in private, maintaining an amazingly united front given the severity of the decisions.

But it will be amazing if some cracks do not appear over the coming weeks, even if the survival instinct is likely to ensure they will all be papered over come 7 December.

The potential fault lines are pretty obvious. Fianna Fáil and the Greens have got on remarkably well over the past three-and-a-half years, given the circumstances, but their relationship is now likely to be put to the test as never before.

And already the future of the proposed Metro North line, between Dublin's St Stephen's Green and Swords, is shaping up as a potential collision point. On RTÉ's Primetime programme on Thursday night, communications minister Eamon Ryan strongly defended the project, arguing it still made economic sense to proceed with it. These comments were echoed by his party leader, environment minister John Gormley, the following day.

Metro North is seen by the Greens as a core issue, part of its commitment to improving Ireland's woefully lacking public transport infrastructure and providing a 'Green future'.

But in Fianna Fáil, there is private bewilderment at the notion that the government could contemplate continuing with the project at a time when it is borrowing €20bn a year to pay the bills. "Can you honestly expect to cut social welfare and build a metro line? You have to live in the real world," was the verdict of one senior government figure.

If this was just a normal budgetary process, this wouldn't be a problem in the short term for the government. It could push out any decision on the metro until well into next year. But this isn't a normal budgetary process. The government has to come up with detailed plans for the next four years, outlining what it is going to spend and what savings are going to be made.

Green for go?

Green party sources may well be right when they assert that "nobody is coming to us to say 'we can't do it'," but it is surely only a matter of time before that conversation on the metro has to take place. Just how the Greens will react if and when that happens is anybody's guess. A metro line is hardly high on the list of many voters' priorities – even on the Northside of Dublin – so pulling out of government on the issue appears unlikely. But the Greens have consistently said they will stay in government only as long as the Green agenda is being implemented.

Just why free third-level fees are part of the Green agenda has never been clear – aside, of course, from the rather obvious point that the Greens' heartland is in middle-class Dublin constituencies. At the Greens' insistence last year, a commitment to maintain free fees were included in the revised programme for government. But some Fianna Fáil ministers believe it is inevitable that the issue will have to be revisited as part of the four-year plan. "How else can you fund third level [in the current climate]?" one Fianna Fáil minister asked, noting that the British government last week lifted the cap on what fees can be charged by British universities. If it's happening in Britain, it seems likely to happen here, even if Green Party sources insist the issue is "sacrosanct".

Property tax

The issues of water charges and a property tax also have the potential to cause problems. Both will certainly be included in the four-year plan but there are problems surrounding their implementation. The Greens insist water charges can only be introduced after meters are installed in every home in the country. The only form of property tax that they will go for is a site valuation tax, which involves valuing every household site in the country. Both these measures could take years to introduce and there will inevitably be pressure to come up with quicker ways of raising revenues. Whether the Greens will be open to compromise on these two core issues remains to be seen.

But the tensions are not likely to be confined to inter-party differences. In previous budgets, a number of senior Fianna Fáil ministers – believed to include foreign affairs minister Micheál Martin and his close ally culture, tourism and sport minister Mary Hanafin – were regarded as favouring a greater emphasis on tax increases rather than spending cuts. Their view did not ultimately hold sway but given the swingeing nature of the cuts planned in December, there may be more support if they decide to take a stand this time around.

The reality is that both big tax increases and massive spending cuts are inevitable. But within those two headings, tough choices remain.

Social welfare rates will be cut across the board, but will the old age pension again be exempt?

There is a huge reluctance within government to touch the pension for political reasons. But privately many senior government figures are wondering for how much longer can the pension be left at its current rate when young people with families and mortgages – both in work and on unemployment benefit – are constantly being asked to make sacrifices.

However you add up the numbers, getting to €5bn is going to be hugely challenging. Presumably, the cuts in the capital budget will rise from the €1bn already pencilled in to beyond €1.5bn and perhaps up to €2bn.

It's difficult to know what savings can come from the public sector pay bill given the Croke Park deal precludes any further pay cuts. But it's hard to see how the €5bn figure can be reached without at least savings of €500m in this area.

The Big Three

Increased taxes are likely to account for another €1bn plus – coming from reduced tax credits, wider bands and, possibly, an increase in tax rates. Indirect taxes are also likely to be raised. But even with this extra tax revenue, it could still leave another €1.5bn-€2bn or so to come off current spending and that's where the discussions around the cabinet table are likely to get hot and heavy.

Much of this burden is likely to fall on three big spending departments of Health, Education and Social Protection. Mary Harney has already said she expects cuts of up to €1bn in the health budget and there is speculation of job cuts of 900-1,300 workers in each of the regions of the Health Service Executive.

Major reforms in social welfare are also likely with options being examined such as a cap on the amount of benefit one household can claim and efforts to reduce the massive budget for rent allowance which has not declined despite plummeting rents in the private sector.

Although there must have been considerable shock in recent days when the €15bn figure emerged, nobody in the cabinet is under any illusions about what lies ahead. But that doesn't mean making the actual decisions is going to be easy.

At the moment, the odds favour the government getting its four-year plan approved and the budget through. But there is a lot of ground to cross before that actually happens. A lot can happen. A lot will happen. Squeaky bum time.

Where Middle-class Voters Could Feel The Pinch
The Areas Under Scrutiny

CHILD BENEFIT

What is at stake?

583,000 families get child benefit which is paid to over 1.1 million children

How much does it cost the taxpayer a year?

€2.15bn

What is it?

A monthly payment to parents or guardians of children aged under 16, and up to 18 if the child is in education, training or has a disability

How much is it worth?

€150 a month for each of the first and second children and €187 for third and subsequent children

Who gets it?

Every family in the country with children regardless of income or assets

What is changing?

The government has looked at means testing or taxing child benefit to reduce the overall bill but protect lower-income families. But both are logistically difficult and an across-the-board cut in the rate looks more likely. The government is also looking at bringing the rate paid for third or more children down to the same level that is paid for first and second children

PROPERTY TAX

What is at stake?

The possibility of introducing a tax on the 1.46 million homes across the country

How much does it cost the taxpayer a year?

It depends on what type of tax is introduced but it is estimated that property tax could easily bring in over €1bn a year

What is it?

The tax-free status of the main family home has existed since the abolition of domestic rates in 1977, although there is stamp duty charged when purchasing a home and a property tax of €200 paid on second homes

How much is it worth?

As the tax doesn't exist, it's difficult to calculate, but the introduction of a tax could mean households paying anything between €200 and €1,000 a year

Who gets it?

The exemption from property tax? Everybody – nobody pays property tax on the principal family home

What is changing?

A property tax is inevitable: if not in the upcoming budget, then as part of the four-year budgetary plan. The government has already committed to introducing a site-valuation tax. While that is being carried out, it might consider the introduction of a flat-rate tax or a tax based on the size of a house as a way of quickly raising revenue

THIRD-LEVEL FEES

What is at stake?

88,400 third-level students are currently exempt from paying third-level fees although they do pay up to €1,500 in registration fees

How much does it cost the taxpayer a year?

€379m

What is it?

Introduced by Labour's education minister Niamh Breathnach, free fees is a universal benefit given to everybody regardless of their economic circumstances

How much is it worth?

Free fees are worth an average of €4,287 per third-level student a year

Who gets it?

Everybody who goes to third level (other than private colleges) gets an exemption from fees

What is changing?

That is the €379m question. The Greens have said they won't countenance any removal of the free fees and Fianna Fáil is not keen on further alienating voters. But with savings of up to €15bn needed – and arguments that it is a middle-class subsidy that is causing significant funding problems for third-level institutions – it is bound to come under consideration

WATER CHARGES

What is at stake?

1.2 million homes are connected to the public water mains supply

How much does it cost the taxpayer a year?

€1bn is spent by the state every year maintaining the water supply

What is it?

While those on private group water schemes pay an annual fee for their water, the vast majority of households are connected to the public system and get that water for nothing

How much is it worth?

This state subsidy is worth around €900 per household

Who gets it?

Free water? Everybody connected to the public water mains supply

What is changing?

The government has signalled that water metering will be introduced in every home and that charges will be brought in for water used above a free allocation. This would bring Ireland in line with the rest of Europe. It is likely to take some time for this to happen, raising speculation about an interim flat-rate services charge. However, this is being strongly resisted by the Greens

PENSION TAX RELIEF

What is at stake?

€1bn – Trinity College economist Michael Collins told last week's Dublin Economic Workshop that savings of up to €1bn could be made by cutting relief on pensions to 20%

How much does it cost the taxpayer a year?

The tax relief currently allowed for private pension provision is estimated to cost the state €2.6bn net in 2007, the latest year for which data is available

What is it?

Tax relief is available for contributions to personal pensions and the amount of the relief is age-related

How much is it worth?

Given that a worker can take earnings of up to €254,000 a year into account for pension tax telief, this is a potentially huge benefit

Who gets it?

People who make contributions to approved personal pension arrangements get tax relief. This relief is more generous as you get older

What is changing?

While currently those paying tax at the top rate can get 41% relief, the renegotiated programme for government includes a proposal to introduce a single 33% rate for tax relief on private pension provision and this single rate proposal is widely expected in the budget. This would directly hit higher earners

TAX CREDITS

What is at stake?

50% of all workers pay no income tax compared to the EU norm of 20%

How much does it cost the taxpayer a year?

The cost to the exchequer of tax credits is €7.8bn in 2010

What is it?

Credits reduce the amount of tax that you have to pay. The tax credits to which you are entitled depend on your circumstances. Current credits are €1,830 (single), €3,660 (married), €2,430 (widowed and one-parent family) and €1,830 (widowed without dependent children)

How much is it worth?

Thousands per individual. Tax is calculated as a percentage of your income. Your credits are deducted from this to give the amount of tax that you have to pay. A tax credit has the effect of reducing your tax by the amount of the credit

Who gets it?

Every worker. Depending on your circumstances, you will be entitled to the single, married or widowed tax credit

What is changing?

It is widely believed that tax credits will be lowered in the upcoming budget. This will bring more low-paid workers into the net but will also mean all workers will pay more tax. Tax bands will also be widened in the budget, ensuring more workers will be paying at the top rate of tax. It is also very possible rates will be increased as an excess of €1bn is sought from income tax.