The holder of the oldest liquor license in Somerville, the Davis Square institution is going on 80 years on Elm Street -- predating the Red Line extension and even most of the old-timers who drink here during the day. On weekend nights the narrow, one-room watering hole packs in the twentysomethings so tight it resembles a T car at rush hour, except everyone’s drinking and no one’s miserable. If you haven’t yet scrawled your name into Sligo’s wooden walls, chances are someone with your name did and you can just lie and tell your friends it was you.