ESB Networks has told the Independent Workers Union (IWU) that it will serve High Court proceedings on the union and relevant officials today in relation to ongoing industrial action by its members - and will notify the Gardaí of any potential incidents of intimidatory behaviour.

The IWU claims to represent over 500 of the company's 1,500 network technicians, and says its dispute centres on management's failure to consult adequately on outsourcing of work to external contractors.

However, the ESB does not recognise the IWU, and has refused to engage with it.

Rival recognised unions put the IWU membership at 300 or less.

For the last week, the union has been engaging in a work-to-rule, with a one day strike last Friday, and a further two-day stoppage scheduled for Thursday and Friday of this week.

ESB Networks yesterday confirmed that the industrial action has had "limited impact" on customers.

In an escalation of the dispute, the IWU has this evening added two extra strike days next week.

However, the union has shortened this week's planned strike from two days - Thursday and Friday - to just Thursday.

Following a meeting of the IWU Strike Committee this evening, it was decided that staff will return their vans to the appropriate depot tomorrow evening at close of business, as demanded by the company.

They will also return their iPads.

Following Thursday's strike, they will return to work on Friday morning, but will again return their vehicles in the evening.

However, their work to rule will continue.

They will then mount a two-day stoppage on Tuesday 4 May and Wednesday 5 May, returning to work at 8am on 6 May.

In a statement, the IWU described the employer's actions as provocative.

It claimed the ESB was trying to draw members into an all out picket and break down their resolve in a "short sharp negative encounter".

The Strike Committee urged members to hold firm.

In a letter to IWU Regional Secretary Gerry Corbett today, ESB Networks Human Resources Manager Glenn Pope reiterates the company's position that the industrial action is being conducted unlawfully - and says High Court proceedings will be served today.

He alleges separately that there may have been intimidatory and harassing behaviour by unnamed parties towards fellow ESB employees and third parties.

"These behaviours include impeding individuals accessing their workplace, persistent pressurising of ESB employees to join IWU, the ongoing use of offensive and intimidatory language towards individuals (via text and in person) and taking photos/videos, without consent, of ESB employees carrying out work duties," he states.

Mr Pope says that he has written to all company staff setting out that these behaviours are not acceptable, and must cease.

"We are reminding staff of our internal policies to deal with Respect, Dignity and Disciplinary issues in the company, and they will be fully utilised, however please also be aware that if necessary we will have no hesitation in notifying such issues to An Garda Síochana."

Mr Pope concludes that the ESB will hold the IWU responsible for "...any loss, damage, harm or injury caused to employees of ESB, our service providers or third parties as a consequence of negligent or criminal acts carried out by IWU members during the course of this unlawful industrial action."

Separately, ESB Networks has told IWU members that they will have to return their vehicles to their allocated base depots by close of business tomorrow ahead of the strike, and will have to make their own way home.

Management notes the continued refusal of IWU members to provide after-hours support involving on-call, stand-by or additional overtime, and their planned strike action.

The company says that home parking arrangements have been permitted based on a "reasonable expectation" that they are available for work, and "provide some flexibilities and overtime as required".

"Based on your stated intent not to attend work on Thursday and Friday next and not to provide overtime cover on Saturday, Sunday and Monday, I am requesting that you park up your vehicle in your allocated base depot by close of busienss on Wednesday next, April 28th.

"The vehicle can be collected at normal start time on Tuesday the 4th assuming you are at work on that day."

The continued use of allocated vehicles for home work and travel will be reviewed after 4 May.

The Secretary of the ESB Group of Unions Jimmy Nolan described the IWU dispute as unnecessary.

"The company is in discussions with the Group of Unions in relation to a new agreement to cover network technicians which could result in an increase in the number of technicians employed, rather than eliminating jobs, as claimed by the IWU," he told RTÉ News.

He dismissed suggestions of potential privatisation as "nonsense".

The ESB has refused to state how many workers from non-IWU unions failed to report to work during Friday's strike.

The other network technicians unions recognised by the ESB - Connect, Unite and SIPTU - have stressed that they are not in dispute with the company and are working normally.

They said that whether to pass an IWU picket would be a matter of individual conscience - but that staff who fail to report for work will not be paid by the company or receive strike pay.