Here is a list of the Frewen that lived in Dublin in 1939
Frewen, Annie 1939-40 RATHMINES EAST GC 175 26 CHARLESTON ROAD
Frewen, Annie 1939-40 RATHMINES WEST HJ 945 14 LEINSTER ROAD
Frewen, Edmond 1939-40 RATHMINES WEST HJ 942 14 LEINSTER ROAD
Frewen, Elizabeth 1939-40 CRUMLIN VA 2350 124 DOWNPATRICK ROAD
Frewen, John 1939-40 RATHMINES EAST GH 155 5 MILLTOWN ROAD
Frewen, John 1939-40 RATHMINES WEST HJ 943 14 LEINSTER ROAD
Frewen, Josephine 1939-40 RATHMINES EAST GC 174 26 CHARLESTON ROAD
Frewen, Margaret 1939-40 RATHMINES WEST HJ 944 14 LEINSTER ROAD
Frewen, Michael Rev. 1939-40 TERENURE YD 417 20 KIMMAGE ROAD, EAST
Frewen, Robert 1939-40 GLASNEVIN CA 1764 8 DE COURCY SQUARE, WEST
Frewen, Sarah 1939-40 GLASNEVIN CA 1763 8 DE COURCY SQUARE, WEST
Frewen, William 1939-40 CRUMLIN VA 2349 124 DOWNPATRICK ROAD
It looks like William that lived in Crumlin was living with a Elizabeth.
Also looks like Julia Frewen got married in 1929. You could request her marriage cert to see what her married name is?
Dublin South, Oct Dec 1929.
Looks like William Jnr died in Oct-Dec 1945 in Dublin South
Looks like William Snr died in Apr-Jun 1915 Dublin South