What if..

It's impossible to say.

For instance in the Civil War would the enormous concentration of power into his hands cause him to develop into a Generalissimo, strangling our democracy at birth to the detriment of the country ever since?
Examples of this abound in other post-colonial countries. Collins' character seemed ill-suited to Bonapartism but the stresses of the Civil War may have changed him. In fact one of the greatest achievements of our first governments was to create a solid democratic foundation for the state, which was far from guaranteed.

Or would he have reined-in the harsher elements of the Free State Government and settled the Civil-War in a more amiable manner, preventing it from being a divisive mill-stone holding back this country and its politics for nearly a century? He engaged in peace attempts prior to his assassination and had an affinity with the rebels not shared by the all of the Free State Government.

From his published writings it seems that he shared Dev's aim of regaining the six counties and turning the Free State into a Republic. It's hard to see anyone surpassing Dev's wiliness in achieving the latter goal, but Collins may well have given him a run for his money. He certainly had the talent.

Economically he seemed to broadly fit in with CnaG/FG policies favouring free trade and investment. So maybe no economic war. But possibly less of the state-driven development that FF performed from the 1930s.