The term 'Fuel for Irish Hearths' refers to PEAT, a type of fuel that is made from partially decomposed vegetation. PEAT is commonly used in Ireland as a source of fuel for heating and cooking. It is formed by the accumulation of organic matter in water-saturated environments, such as bogs and marshes. PEAT has been used as a fuel source in Ireland for centuries and is still used in some parts of the country today.