Il Mario's restaurant it is found at Naoussa, Paros, right in the main square where it was also filmed The Journey is immature. This place, very nice and overlooks this delightful square where there are also many other restaurants.
The staff speak a little Italian, like many of the Greeks actually, and eating here is useful for those who don't know any English. Eating at Mario's means eating Greek dishes but also some Italian reinterpretations, such as the spaghetti with olives and feta, really very good.
If you are looking for a typically Greek meal however I don't recommend this place, given the central location it is a little tourist, you should instead take off in the secondary lanes of Naoussa. otherwise the place is valid and the dishes good, nothing to say.