After his father died, John started hanging out at bars.
Literal
Father [subject-が] passed-away [then-て], John [topic-は] bars [to-に] frequenting came-to-do.
入り浸る means 'to hang around constantly, to haunt (a place).' ~ようになった (came to do) marks the start of a new habit. 居酒屋 (Japanese-style pub) is culturally specific.