Interesting article here from John O'Donovan on the All For Ireland League - a nationalist party based mainly in Cork.
The All-for-Ireland League 1909-1918 | The Irish Story
They split from the IPP due to them being too sectarian and not conciliating the Ulster unionists.
The IPP especially the Hibernians obliged them by trying to beat them off the streets of Cork in the 1910 elections. There were even shots fired.
William O'Brien had always maintained that the IPP were too confrontational but puzzlingly enough (to me anyway) they threw in their lot with SF in 1918.