Croke Park is only asking U2 to foot a fraction of the cost of replacing its hallowed turf after their concert there because algae are killing the grass anyway.

The pitch has been dug up in advance of the band's three Croker gigs as part of the band's 360 degree Tour later this month.

But bosses at the stadium said it would be unfair to hand the whole bill for replacing the pitch to Bono and the boys because it needs replacing anyway.

While U2 are only being asked to pay 30pc of the €1.2m cost of the project, stadium director Peter McKenna said this was the fairest way to split the bill.

He said: "The way we look at it is, we would have to replace our pitch in 2010, because of an algal layer that is about 5cm under the surface.

"In winter time, the grass roots are going into that level and the pitch is dying off.

"With the concerts, we have been given the opportunity to move the timetable forward and do the work now.

"What we would have spent in 2010 is reduced substantially. It would be unfair to pass the full cost on to U2."

Mr McKenna explained that the algal problem was common in stadia where part of the pitch was in shade. It meant the grass had to be replaced every seven to 10 years.

Sand is currently being spread across the surface in preparation for the stage and thousands of fans that will arrive for the concerts.

Sods will then be grown in Britain, harvested and planted in Croke Park in advance of future matches. The stadium has already had to defend itself over criticism that matches will be played "on British soil".

A convoy of heavy machinery from Britain is on the pitch working on the project, with an Armagh-based company contracted to do the work. The sods are being sourced in England and British companies are almost exclusively providing pitch advice, seeds and turf.

Fianna Fail senator Mark Daly criticised the sourcing of a British turf expert, saying it sent out a "symbolically terrible signal". Mr McKenna pointed out that Ireland does not have the specialist turf farms or grass seed nurseries that can provide Croke Park with what it needs.

He said: "We're not playing games on British soil. We need to see the bigger picture. It's a strange way to look at it."