Gaelic Storm has straddled the line between tradition and innovation for nearly two decades. Led by founding members Patrick Murphy and Steve Twigger, the group infuses traditional Celtic music with modern influences, updating the genre for a new generation of fans raised on rock, country and folk. Along the way, the musicians have topped the Billboard World Chart four times, appeared in the blockbuster film “Titanic” (where they performed “Irish Party in Third Class,” an unreleased song made available for the first time on Full Irish) and earned a reputation as a hard-touring, genre-bending band. Regularly playing more than 120 shows a year, Gaelic Storm has built a loyal following as diverse as the band’s own music. Fans of traditional Irish music have championed the group for years, but so have fans of harder-edged Celtic rock.