The Irish League is ranked 47/54 with uefa league rankings. Malta, San Marino, Gibraltar Faroe Islands etc are ranked below it.
The Irish League is and will remain a part time league, there is no tangibile or realistic chance of the top clubs progressing.
The current supporters will continue to support Crusaders, Coleraine Ballymena Glentoran etc but the fact you mention the Belfast clubs attract 6,000 supporters, shows how poorly the Irish League is supported.
If a club was up and running in England, there would be limited support initially, but by marketing it correctly, getting into the schools, working closely with the local underage leagues and giving them exposure, there is a hell of a lot of room for supporting a local team.
The key would be too get back to back promotions quickly, like Guernsey fc done, AFC Wimbledon, Utd of Manchester fc etc, suddenly then you have momentum, the media want a good news story, you have FA cup fixtures etc, supporters will get interested.
Man Utd and Liverpool fans will still go across the water once a year, to see a premiership game, but they will still be in Belfast the rest of the year and if there is a local team rising through the non leagues in England I believe it will attract the attention of a lot of them.
As you said Belfast is a football mad city, the people in Belfast need a team they can get behind, at the minute the Irish League doesnt do that and is unlikely to ever do it.