"What motivation do you need to play for your country? I'm fit and ready and available for selection," McClean told reporters on Tuesday.
"It gets on my nerves when players come in and they're not fit for games, but they are for the club games at the weekend. That gets to me.
"To see the excitement here of lads who want to wear that jersey, it's brilliant."
After the Mexico match, O'Neill's squad will head to Dublin where they host Uruguay in another friendly before their Group D World Cup qualifier against Austria on June 11.
(Reporting by Aditi Prakash in Bengaluru; Editing by Peter Rutherford)
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