The High Court has reserved its judgment in a case taken by the Dublin Airport Authority and Ryanair seeking to prevent SIPTU's planned four-hour strike on Friday.

Mr Justice Paul Gilligan will deliver his ruling tomorrow.

The strike could shut down the three State airports and Aer Lingus on one of the busiest travel weekends of the year.

Lawyers for the DAA told the court that it will suffer catastrophic and irreparable damage, as well as losses in the "millions of euro", if the strike goes ahead.

The DAA and Ryanair are seeking an injunction to prevent the stoppage.

Mark Connaughton SC told the court that over the four-hour strike period, 10,000 passengers were due to fly in and out of Dublin Airport, while many more would face cancellations, as knock-on disruption would continue for up to 24 hours.

He estimated that the losses would run to millions of euro, but that SIPTU was unlikely to be a "mark" for recovering those damages.

Mr Connaughton challenged the validity of SIPTU's ballot of members at the DAA.

He also said that certain key personnel, including the airport fire and police service, as well as the search units, were subject to agreements that obliged them not to take industrial action until all procedures were exhausted.

Mr Connaughton said that the pensions dispute was really a dispute with the pension scheme trustee, and as such was not a valid trade dispute for the purposes of union immunities and protections applying under the Industrial Relations Acts.

He told the court that the balance of convenience lay with granting the injunction.

Ryanair counsel Martin Hayden said his client would suffer extremely grave consequences from being "caught in the crossfire".

Mr Hayden accused SIPTU of blackmailing their employers and taking the nuclear option of a strike, which would bring them back to the Victorian era.

He said the union could not decide to blow everyone up and to hell with the consequences.

However, SIPTU's counsel Richard Kean strongly defended the union's entitlement to take strike action and accused Ryanair of being inflammatory.