It's an awfully large presumption to jump from the information in that article to "Shane offered him money to fuck him (Shane) in the ass."
Assuming that the statement about solicitation is true, I'd guess that most johns prefer that the prostitute be on the receiving end.
Regardless, this is clearly a case of "he said, he said." The only thing known for certain is that young Mr. McInerney assualted Mr. MacGowan in the pub's toilets. What led to the attack is clearly open to debate since the only two participants have provided conflicting information. For anyone of us to speculate on it is beyond silly since we have no more information than the conflicting stories.
Regardless, who actually cares? At the end of things it doesn't matter what Shane's sexual preferences are. The proper response for an unwelcome solicitation is to walk away, not assault the john with a bar.
“I know all those people that were in the film [...] But that’s when they were young and strong and full of life, you know?”