You'd need billions and resign yourself to losing most of the money you invest.
I'd buy every club in League of Ireland and start restructuring and changing the way they operate. I'd create under-age structures in all the clubs, and I have an agreement between the sides, where they would play each other regularly from the youngest age group to under 16 without any league tables or trophies. It would be purely a talent development league.
The senior league, I'd build 12 new 10,000 seater stadiums, which could be expanded. I'd force clubs in close proximity to share. I'd then create a League of Ireland TV Station running 24-7. I'd create a web presence that would be unrivalled in it's comprehensive coverage. I'd make sure the games where covered with multiple camera points and force the public to feel like by not going to a game was to be missing out.
In summary, I'd throw as much cash to ensure the standard of player was second to none and that the league was unmissable :
LOL