Well, still struggling.
12A - Hero of Irish novel posthumously published (7) I have S?E?H?N
16A - Upset a north-eastern crime writer (8) I have ???Y?A?E
17D - Early viticulturist and good-time Charlie (8) I have ?I?N?S?S
19D - Fellow in agony could be Ganelon (7) I have ??L?D?N
For 3D I have Southerly (the poet being Southey) but not sure.