U2 is adding eight shows to its opening run at James Dolan’s new futuristic Las Vegas Sphere arena — even as fresh cost overruns threaten to deepen the mega-buck construction project’s financial hole, The Post has learned.

The legendary rock band most recently had been scheduled for 17 shows as the first act at the $2.3 billion, globe-shaped arena that looms over Las Vegas and which is slated to open in September.

As reported by The Post, billionaire Knicks owner Dolan has quietly already agreed to pay the legendary rock group $10 million to produce the splashy show.

The Sphere ran an ad during the Super Bowl promoting U2’s opening appearance.

Aside from the 25 U2 gigs, sources told The Post this week there still are no additional acts scheduled, although Dolan is now in talks with Madison Square Garden’s favorite jam band Phish and is close to a deal.

This summer Phish is appearing seven nights, July 28 through August 5, at Madison Square Garden.

Phish is known for hosting festivals, and would seem more open than other bands to designing a show that fits with the 17,500-seat arena and its 160,000-square-foot LED screen, a source said.

Dolan also is speaking with Coldplay about a possible series of shows, according to the sources.

Dolan is scrambling to fill gaping holes in the Sphere’s calendar after disclosing in a Wednesday morning earnings report that the still-unfinished venue’s construction costs — which were initially $1.2 billion and had ballooned to $2.175 billion — are now expected to total “approximately” $2.3 billion.

Shares of Sphere Entertainment — a new, publicly traded spinoff from Madison Square Garden Entertainment that owns the Las Vegas project — have tumbled nearly 18% to $24.51 at Thursday’s close following the announcement.

Morgan Stanley in an April 4 analyst report predicted that Sphere Entertainment would trade after the spin-off at $37.20.

The Sphere’s soaring construction tab mainly reflects the overall complexity of the project, Dolan’s Sphere Entertainment Corp. said on Wednesday.

“The only thing that matters in their earnings is the cost overruns,” an analyst requesting anonymity said. “No one exactly knows when there will be an end to it.”

Dolan has also not shared a detailed operating plan for earnings from the Sphere. The Sphere does not have a naming sponsor, for example, sources said. Potential sponsors are waiting to see the behind-schedule concert lineup, a source added.

The Knicks and Rangers owner has much of his personal fortune tied up in his new arena which has become his passion project, sources said.

Dolan on April 20 split MSGE in two. He completed the spin-off of the rest of its live entertainment business including Madison Square Garden, Radio City Music Hall, and The Beacon Theatre and then renamed the parent company Sphere Entertainment, which consists of The Sphere and the MSG sports cable network.

He owns his stake in the teams through the listed company Madison Square Garden Sports.