Context: The split effectively ended the realistic electoral chances of the Irish Unionist Alliance in southern Ireland. As the partition of Ireland became more likely, Southern Unionists formed numerous political movements in an attempt to find a solution to the "Irish Question". Among these were Irish Dominion League and the Irish Centre Party. As such, the southern rump of the IUA became increasingly fractured and in 1922 it lost its reason to exist with the establishment of the Irish Free State. Leading unionist figures, such as the Earl of Midleton,  Lord Dunraven, James Campbell and Horace Plunkett were appointed in December 1922 by WT Cosgrave to the Free State's first Senate. Amongst others, Horace Plunkett's home in County Dublin was then burnt down during the Irish Civil War  because of his involvement in the Irish Senate. The IUA helped form the Southern Irish Loyalist Relief Association to assist war refugees and claim compensation for damage to property. From 1921 IUA voters began to support the mainstream Cumann na nGaedheal party. In the 1923 election three formerly loyalist businessmen were elected as the Business and Professional Group. From 1921 to 1991 the proportion of Southern Irish Protestants declined from 10% to 3% of the population; these had provided the bulk of the IUA's support base. Unionists continued to have a majority on Rathmines Council until 1929, when the IUA's successors lost their last elected representatives in the Irish Free State.

Question: The ratio of Southern Irish Protestants declined from 10% to 3% over how many years?

Answer:
70