Shell's Corrib gas project has been delayed for years by strong resistance in County Mayo. Now claims are emerging of corporate sweeteners, including a consignment of alcohol for police after a clash with protesters

For 10 years, the Shell oil and gas behemoth has endeavoured to bring ashore a pipeline from the Atlantic into the heart-stopping beauty of Ireland's County Mayo seaboard. And for 10 years, local people whose ancestors farmed the land and fished the ocean have been determined to stop it.

The struggle has become an epic clash between the Goliath that is Shell, backed by the Irish police, and a group assembled around the umbrella protest group Shell to Sea, whose founder, retired primary schoolteacher Maura Harrington, says that, "thanks in no small measure to the Shell to Sea campaign, the project is 10 years behind schedule and its budget has trebled".

An internationally award-winning film, The Pipe, directed by Risteard O'Domhnaill, has vividly charted the confrontation on the little rural strands; farmers and fishermen beaten and jailed; riot police and balaclava-clad guards mobilised across little lanes winding through bog to the brine.

But beyond these surreal scenes, a stranger battle rages as Shell struggles for the "hearts and minds" of the community, using what the senior press officer for the company in Ireland, John Egan, calls "accommodation services" – sweeteners, or "donations", to the people of the region.

The campaign to win the residents' goodwill has sometimes displayed dashes of Whisky Galore-style farce – shower facilities for the local football team and suchlike. But it has also become a morality play of sorts, shedding much light on how the cogwheels of big corporate influence turn in a remote rural community.

And the "accommodation services" have become the focus of a bitter dispute between Shell and a tiny oil services company, OSSL, which claims that it was charged with "greasing the palms" of interested parties on County Mayo's coastline. The company, managed by Desmond Kane from Glasgow and Neil Rooney from Belfast, insists that the services it carried out for Shell even ran to providing the police force with alcohol soon after a major clash with protesters – along with other outlandish favours to residents.

More sinisterly, OSSL also claims that a Shell manager demanded that Rooney withhold evidence after the clash, which occurred at Pollathomais in 2007. Rooney says that he heard an officer say of the pipeline protesters, "drive them into the sea", but was told that this "cannot be part of your statement" to an ombudsman because the officer concerned was "our man" and "had to be protected at all costs".

OSSL's allegations come in the wake of a recent report by a special rapporteur for the United Nations which found "excessive and disproportionate use of force against protesters" peacefully opposing the pipeline.

Rapporteur Margaret Sekaggya said that she "received credible reports and evidence … indicating the existence of a pattern of intimidation, harassment, surveillance and criminalisation of those peacefully opposing the Corrib gas project … Moreover," she reported, "there have been serious concerns about the lawfulness of certain actions of the private security firm employed by Shell."

The rapporteur "expresses her concern at … shortcomings in official investigations, particularly those relating to the use of excessive force and abusive behaviour by the police".

OSSL had served Shell since the oil giant took over the Corrib pipeline in 2002, and had been contracted onsite to Shell's predecessor, Enterprise Oil. Kane and Rooney describe having "to provide whatever's needed, whatever time of day or night. If they needed 100 fireproof gloves at 11pm, it was our job to get them. We did a good job, Enterprise told us they'd never seen such service in Aberdeen, and Shell kept us to carry on that level of service."

But the "accommodation services" went too far for OSSL. It was tasked to provide "a tennis court, cookers, television sets, agricultural equipment, school fees, home improvements, garden centre visits, forestry equipment", says Rooney – for local residents. He says that he and Kane found themselves paying workmen to do one thing, then invoicing Shell for something else, and often administering "accommodation services" themselves.

The pattern was the same as the saga reached its reported nadir: the delivery, from Northern Ireland in an unmarked van, of alcohol worth €35,000 (£30,100) to the Garda station at Belmullet, where the policing operation was quartered at Christmas, 2007. Kane quotes a Supt John Gilligan as saying, while he was helping to unload the consignment of booze, "it's lucky these walls are high", lest the protesters caught a glimpse of what was going on.

Arguments eventually developed over invoicing the "accommodation services", with Shell allegedly asking that another contractor be invoiced instead of the oil giant directly. Relations worsened, and OSSL's contract was ended in 2010.

OSSL sought to recoup payments, including the outlay for the officers' alcohol. Kane pleaded to Gilligan in February 2011: "I write to ask if you can assist us … At the Christmas period of 2007, we were instructed by Shell E&P Ireland to purchase and deliver festive gifts to … Belmullet Garda station." Shell has "failed to reimburse us for the outlay". He adds: "At Shell's insistence, these gifts came with a high degree of confidentiality, which we have adhered to until this very day."

Gilligan was transferred to become a press officer at the Garda Síochána headquarters in Dublin, where he continued to correspond with Kane, telling him to register the grievance with his successor at Belmullet.

Shell and OSSL settled in 2011, with Kane saying that to contest Shell's vast legal war chest would bankrupt him. Shell, which employed the accounting firm KPMG, called the agreement "a full and final settlement". OSSL continued to barrage Shell with emails pleading its cause, until the chairman of Shell Ireland, Michael Crothers, replied to say that "I personally felt some moral obligation to try and find a way to a settlement."

Shell and the gardaí fall shy of denying outright OSSL's allegations. Denise Horan of Shell issued a statement to the Observer saying: "OSSL alleged falsification of invoices and further alleged delivery of alcohol to the gardaí, as well as non-business-related works and gifts … The investigation team did not find any evidence to support OSSL's allegations."

The gardaí's written statement was similar: "Allegations were made to the district officer at Belmullet that alcohol was distributed to members of An Garda Síochána on behalf of Shell E&P. Inquiries conducted in relation to these allegations found no evidence of alcohol being distributed to members of An Garda Síochána by, or on behalf of, Shell E&P."

The Observer asked the Garda and Gilligan specifically to deny the delivery, but no denial came, only a repeat of the prepared statement.

The Observer replied that, unless a denial was received, we would presume the veracity of OSSL's story. Silence ensued.

Much correspondence between Shell and OSSL is posted on a website which then came to play a key role: royaldutchshellplc.com, run by John Donovan and until recently his father, Alfred, who died last month. The site, a thorn in Shell's side, is a watchdog on the company and repository for material leaked by whistleblowers and discontents, with more than 30m monthly hits.

The Donovans had secured places for Kane and Rooney at Shell's annual meeting last month, to raise their grievances. Cornered, the company's CEO, Peter Voser, suddenly ordered a further inquiry, a move echoed by the Garda.

But still OSSL waits for any fresh meetings to take place, and still the mystery of the corporate oil giant, the local police force and a large consignment of alcohol goes on against the backdrop of one of the most breathtaking shorelines in Europe.

Maura Harrington of Shell to Sea, the only person to be tried and convicted after the clash at Pollathomais, said last week: "There's a chasm between Shell's rhetoric of its stated principles of corporate governance and the reality of Shell tactics on the ground. In the failed state that is Ireland at present, Shell is given free rein to do as it wishes."

The whole matter has been raised by an Irish TD, Clare Daly, who received the same platitudes as the Observer, and now says: "If there was no foundation to the claims, then surely there would have been a vociferous denial … It's time that the minister for justice steps forward and demands some answers. The issues at stake are too serious to let it fester.

"There have been huge questions around the policing of the Corrib gas project, with many people believing that An Garda Síochána has been behaving like a private security company for the very wealthy Shell corporation, while trampling on the rights of residents in the area."

• This article was amended on 12 August 2013. The headline was corrected to use the plural term for the Irish police rather than the singular.