"There is a 'session' every Tuesday night at St. John Stones Social Club, Sandbrook Way, Ainsdale from 9pm to 11ish. Mainly Irish folk music, but a bit of everything really. Has 'Celtic Spirit' as resident musicians. All welcome."
I haven't been to these sessions myself yet, but I'm not aware of anywhere else in Southport where Irish folk is regularly played, so if that's your bag, give it a try. The venue doesn't sell real ale, but I expect Guinness would be the favourite anyway.