Question: How many values of r z(arcsecond) are associated with a target datum of Ireland 1965?
Schema: create table table_15318324_1 (r_z___arcsecond__ varchar, target_datum varchar, PRIMARY KEY (r_z___arcsecond__))
SQL: select r_z___arcsecond__ from table_15318324_1 where target_datum = "ireland 1965"