Murphy is the anglicized face of Irish Ó Murchadha (and related Mac Murchadha): “descendant of Murchadh.” The personal name Murchadh is usually explained as “sea-warrior” (muir “sea” + cath “battle”). So the surname is a parent’s-name / descendant form, not an English word about bad luck.
Several distinct Irish lineages used Murchadh-based surnames; anglicization collapsed them into Murphy on English records. That is why the name is Ireland’s most familiar export surname without implying one medieval couple for every Murphy household. Family-sendable version: Irish “descendant of Murchadh,” with the old given name carrying a sea-fighter sense.