In a city full of Irish bars, Foley’s rises to the top like a layer of frothy foam on a pint of Guinness. Billed as Boston’s oldest continuously family-owned pub, it’s stayed adaptable: Once the domain of gray-haired, ruddy-nose politicos and eavesdropping reporters from the formerly neighboring Boston Herald headquarters, Foley’s now draws trendy new South Enders and occasional surprise guests—Justin Timberlake tossed back a few here post-concert—with suds, grub, heart and history.