The claim, lodged in the High Court last month, follows a €10m Cab settlement with Tom McFeely, a former IRA hunger striker turned property developer.

A High Court order last month also gave the Cab permission to keep the €1m in cash and cheques gardai seized during the cross-border search of Murphy’s home last March. Murphy, the former chief of staff of the IRA, has yet to reply to the civil summons and has not instructed lawyers.

The tax bill — an early “guesstimate” of the proceeds of almost 30 years of fraud and crime by the Louth farmer — can only be challenged by way of an appeal.

The onus is now on Murphy, who built a multimillion-euro empire through fuel smuggling, money laundering and property deals, to challenge the bill through the courts. The alternative is to make a private settlement with the bureau.

Murphy’s empire has unravelled ever since he lost a libel action against The Sunday Times in which he denied being a member of the IRA. Gardai and the PSNI are now determined to stamp out criminal activity in the republican movement in advance of a political settlement in Northern Ireland.

Gardai say the €5.3m bill is an “initial instalment”, and it could increase hugely as the investigation into Murphy’s affairs continues. “This is only the beginning,” said a senior investigating officer who participated in the raid. “Slab has never been convicted of any crime, but a civil case like this will hit him where it hurts. It is a stroke of genius.”

During the raid, the Cab served an income tax assessment notice on Murphy in his absence. It was placed on his kitchen table, beside his half-eaten breakfast.

Meanwhile, other officers found cash and cheques in plastic bags in a hay shed. Up to 12 vehicles, about 30,000 cigarettes and a large quantity of fuel suspected were seized. There were also two shotguns and chemicals that could be used for doctoring diesel.

Murphy, who slipped the security cordon and fled after getting a few minutes’ notice of the raid, had 28 days to appeal the assessment notice or settle his affairs with the Cab. He failed to do so, resulting in last month’s court order.

Under the TCA, the burden rests on any person appealing an income tax assessment notice to prove that the finding is inaccurate.

They must do this by giving sworn evidence in a witness box — declaring their true income and filing returns. This prospect is likely to hold little appeal for the notoriously secretive Murphy.

Officers from the Cab, who filed the action in the High Court on April 24, have attempted to contact several legal firms who have previously acted for Murphy in an attempt to settle the claim.

While it has been widely reported that Murphy has gone to ground since the raid, the Gaelic football fanatic has actually been staying with family and friends and has been spotted at fixtures involving Naomh Malachi, a Gaelic football team based in Sheelag.

He has also been spotted in Dundalk by garda surveillance officers.

Murphy is the second high-profile republican entrepreneur to be targeted by the Cab this year. The agency recently settled a €10m claim with McFeely, a key player in the €500m extension of The Square shopping centre in Tallaght.

The former IRA member served 12 years of a 26-year sentence in the Maze prison for robbing a post office and shooting and wounding an RUC officer during a siege of a house in Co Derry. Now resident on Dublin’s upmarket Ailesbury Road, McFeely worked his way into the Square deal through a shrewd property manoeuvre.

After he was convicted by the Belfast City Commission Court in 1977, a judge told McFeely: “You are a danger to the public and a greater danger to the police. It is clear that you must be put away for a long time.”

McFeely replied: “I may serve the term, but you will not.”

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