Gardai are investigating a suspected arson attack at a mobile mast site in Letterkenny which was being upgraded to help a local hospital.
The attack, which saw two fires being lit, is believed to be related to a spate of 5G conspiracy theories currently being circulated in relation to Covid-19.
An Eir spokesman said the masts that were targeted do not have 5G capability, but are instead updates to existing 4G infrastructure to "help indoor coverage at Letterkenny Hospital and the surrounding area".
Both fires were extinguished by the fire service and no injuries were reported.
Gardai said no arrests have been made and their investigations are continuing.
The incident comes after attacks on dozens of 5G masts in Britain, with conspiracy theories linking 5G to the coronavirus and other health hazards.
The World Health Organisation (WHO) recently published an advisory notice specifying that "5G mobile networks do not spread Covid-19...viruses cannot travel on radio waves".
However, last week, protesters threw onions at mobile engineers upgrading a 4G mobile site in Ballyfermot.
The work was being done by the telecoms contracting firm KTL.
"It was quite dangerous," said the company's chief operating officer, Peter Dwyer.
The attacks come as communications networks are under growing pressure to maintain capacity and service quality during the lockdown.
Last week, the telecoms regulator approved additional spectrum capacity for mobile operators to help vital services and people working from home.
However, the conspiracy theorists insist that 5G damages health through radiation, with many spreading rumours online linking it to Covid-19.
Medical, regulatory and scientific bodies have overwhelmingly rejected such claims.
"There is simply no evidence to support worrying about phones or masts when talking about cancer risk," according to a guide by the Irish Cancer Society on the 5G health question.
This was echoed by Ireland's telecoms regulator.
In the UK, This Morning host Eamonn Holmes was branded "irresponsible" for comments he made on 5G and Covid-19.
He spoke out on the ITV show after presenter Alice Beer branded the conspiracy theories that link them as "ridiculous" and "incredibly stupid".
Holmes (60), who was co-presenting with his wife Ruth Langsford, responded that "it's very easy to say it's not true because it suits the state narrative".
He told Beer: "I totally agree with everything you're saying, but what I don't accept is mainstream media slapping that down as not true when they don't know it's not true.
"That's all I would say as someone with an inquiring mind."
He made the remarks after scientists dismissed any link, calling it a "physical and biological impossibility" and branding conspiracy theorists "a public health danger".
Viewers criticised Holmes and accused him of "legitimising" the conspiracy theories.
@vickster51 wrote: "So I've just heard This Morning presenter Eamonn Holmes legitimise the stories linking the coronavirus lockdown to 5G.
"On national television. Wow. How irresponsible can you get?"