Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_15318324_1 (r_z___arcsecond__ VARCHAR, target_datum VARCHAR)

### Question ###
How many values of r z(arcsecond) are associated with a target datum of Ireland 1965?

### Accurate SQL ###

SELECT r_z___arcsecond__ FROM table_15318324_1 WHERE target_datum = "Ireland 1965"