Differences over whether Ireland should have complete fiscal autonomy or the customs and excise duties to be retained by the Imperial Parliament then came to the fore and threatened to completely disrupt the Convention. The bishops held rigidly to full fiscal autonomy for an Irish parliament. Bishop O'Donnell proposed a plan for Ulster Unionist of an additional twelve nominated representatives in an Irish parliament, and a further twelve for the Southern Unionists, limited to ten years and a joint British-Irish Commission to negotiate a customs union between the two countries. From the moment the "O'Donnell scheme" became the basis of discussion, provincial federalism ceased to be one of the possible options before the Convention.