Irish Citizenship by BirthAnyone born in Ireland prior to 1 January 2005, except for children of parents holding diplomatic immunity in Ireland, are automatically granted Irish citizenship. You are also automatically considered an Irish citizen if you were born outside of Ireland between 1956 and 2004 to a parent (mother and/or father) who was an Irish citizen born in Ireland. A person born in Northern Ireland after December 1922 with a parent or grandparent born in Ireland prior to December 1922 is also automatically an Irish citizen.
Even if you always assumed that your grandparents were English, you might want to check their birth records to learn if they really meant England - or if they were possibly born in Ulster, the province which is Northern Ireland. Although occupied by the British, the Irish constitution claims Northern Ireland to be part of the Republic of Ireland, therefore most people born in Northern Ireland prior to 1922 are Irish by birth. If this applies to your parent or grandparent, then you are also considered to be an Irish citizen.
Irish Citizenship by DescentThe Irish Nationality and Citizenship Act of 1956 provides that certain persons born outside of Ireland may claim Irish citizenship by descent. Anyone born outside Ireland whose grandmother or grandfather, but not his or her parents, were born in Ireland may become an Irish citizen by registering in the Irish Foreign Births Register (FBR) at the Department of Foreign Affairs in Dublin or at the nearest Irish Embassy or Consular Office. You can also apply for Foreign Births Registration if you were born abroad to a parent who, while not born in Ireland, was an Irish citizen at the time of your birth.
There are also certain limited circumstances where you may be eligible to obtain Irish citizenship through your great-grandmother or great-grandfather. This can be a bit complicated, but basically if your great-grandparent was born in Ireland and your parent used that relationship to register as an Irish Citizen by Descent by the time of your birth, then you are also eligible to register for Irish citizenship. Citizenship by descent is not automatic and must be acquired through application.