James McClean is also from Northern Ireland, I presume the lad is of a catholic background and prefers the Tricolour as a result, rather than a faux-pas by the admin.
An 'International Club' is any club not affiliated or registered with the FA. Glentoran are registered with the (Northern) Irish Football Association so presumably that's why he needed international clearance. He's played for Northern Ireland u17s so I think it's quite likely that someone has got the flag wrong, in the same way that they spelt Glentoran wrong.