I've parked at the Oval with a southern registered car a few times and never had a problem incl an U19 UEFA qualifier NI v RoI and the Shels champions league game there.
It is not in the nicest part of Belfast by any means. Best to park early close to the main entrance and walk around to the away end (bit of a walk).
You could park in a city centre multistorey and get a bus/taxi if you are really concerned.
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