Regarding the Boar's Head's Folk Session and Poetry & Pints/Open Mic nights; they've both been going for years and they're both very relaxed, friendly and informal evenings.
There's a lot of crossover between the two events and the only real difference is the P&P/Open Mic night, as the name implies, gives people the option of using a (very good) PA system and is a little more structured, so that we can ensure that everyone gets a fair go.
Both events will also be major fringe events at the Folk & Boat Festival in June.