Celtic Thunder, Washington’s best-known Irish band, has played twice at the White House. Now it brings jigs, reels, ballads, poetry, and dance to its 20th-anniversary concert at the Museum of Natural History. Combining traditional songs and original compositions, the group has shown the diversity of Irish and Irish-American culture with whimsically romantic ballads and dexterously arranged dance tunes. Terry Winch contributes button accordion and writes many of the group’s songs, including his standard, “When New York Was Irish,” which has been recorded by more than 15 bands. Washington step-dance instructor Regan Wick will perform original dances and plays a deft, rhythmically vital piano. At 7 p.m. at the Museum of Natural History’s Baird Auditorium, 10th & Constitution Ave. NW. $19. For reservations call (202) 357-3030. (Mark Longaker)