The likelihood of a Third Tier are the following being involved;
STRONG:
Klub Kildare
CK United
Mayo FC
Shamrock Rovers II
UNKNOWN:
Shelbourne II
St Pats II
Derry City II
Galway United II
Waterford II
Bohemian II
Cork City II
At least 10 of the above 11 will be needed for a national league. There isn't a queue of non LoI clubs joining the academy leagues. Clubs joining the academy leagues is more a sign of dependability as well. Cavan Monaghan have come and gone. Better to happen at academy level than to damage the brand of a Third Tier trying to get off the ground and become established.
A Third Tier could get by with 8 but if anyone pulls, less than 8 is far from ideal.