Presuming you have data like this in your geojson:
{"type":"Feature","properties":{"name":"Ireland"},"geometry":
{"type":"Polygon","coordinates":[[[-6.197885,53.867565],[-6.032985,53.153164],
[-6.788857,52.260118],[-8.561617,51.669301],[-9.977086,51.820455],
[-9.166283,52.864629], [-9.688525,53.881363],[-8.327987,54.664519],
[-7.572168,55.131622],[-7.366031,54.595841],[-7.572168,54.059956],[-6.95373,54.073702],
[-6.197885,53.867565]]]},"id":"IRL"},
you would take the properties of the object and define parallel properties in your model definition.
Then you easily take your geosjsonobject in your model constructor:
var Ireland = new MyCountryModel(my_geojson_object);
To take care of parsing JSON you may be getting from the server, you define a method parse(response)
in your model and/or collection definitions, which takes your server response object(string) as argument, and should return JSON
object ready for use in initialization of collection/model