I know, I know, another league structure post, but I have often thought about an NFL style conference system working for a regionalised 1st division.
Here is the new Rugby Pro 14 format:
https://youtu.be/j-5FhbB4Kp8
Entice Monaghan, Kildare, Kilkenny back. Add a Kerry league team and any other regional team. Allow Premier teams to have reserve/u23 teams at a reasonable cost. And hey presto. Enough teams to have a regionalised low cost 1st division.
Increase the premier to 14 and run it like the Belgian league and you have plenty of variety of fixtures.
League solved.