President Reagan will be pleased to learn that one of his favorite authors, Morgan Llywelyn, has returned to Annapolis, where she has lived much of her life. Llywelyn is the author of "Lion of Ireland," the story of the Irish leader Brian Boru, who is considered the first and only man to unite Ireland. The president supposedly can trace his ancestry back to the 10th-century leader. Llywelyn sent the president an autographed copy of the book and he became an avid fan.

He invited her to lunch at the White House and to the last inauguration. He has also read her later book, "The Horse Goddess," and is said to be eagerly waiting for her new book, "Grania," about a 16th-century Irish woman pirate named Grace O'Malley. Like Boru, O'Malley actually existed; she was a contemporary of Queen Elizabeth I. The book will be published, fittingly enough, on St. Patrick's Day. Now after living for four years in New Hampshire, where her husband of 28 years died, Llywelyn says she is back home in Annapolis to stay. Red Alert! Red Alert!

Cosmopolitan magazine's bachelor of the month, Steve Varsano, has lost his little black book, except his book's not so little. Varsano says "my big black book" disappeared a little over a week ago in Washington or Philadelphia. He has placed ads in papers in both cities offering a $1,000 reward for its return. In addition to the names and addresses of some 200 women, Varsano said the book also contained phone numbers of some important business contacts. Some of us don't even know 200 women! Poor Steve will have to start over on his collection.

But even without his big black book, Varsano didn't go without a companion this weekend. While the Redskins' Joe Theismann was about town with Cathy Lee Crosby, his ex-wife Shari Theismann was the hit of the Food for Christmas ball at the Shoreham Hotel with Varsano. But when asked about his relationship with Shari Theismann, Varsano gave the professional bachelor's standard response: "We're just good friends." End Notes

Royal Watch, in Retrospect: It seems that Princess Diana wore only red, white and blue while in Washington. She arrived in a red suit with a white shawl collar. She went to the White House dinner in midnight blue velvet. She attended the Washington Cathedral and toured the National Gallery's "Treasure Houses" exhibit in a tailored white and navy suit. That night at the British Embassy dinner she was in cream taffeta and lace. On Monday she wore a ruby red blouse and accessories with a tight white suit. At Arlington Cemetery, she wore a deep blue dress with black cummerbund. And at the National Gallery dinner that evening, a sparkly silver beaded dress. Not white, but close enough. When she left on Tuesday for Palm Beach, with Washington behind her, she changed her color combinations. She was in a hot pink jacket and black skirt. Nigel Elacott, the information officer at the British Embassy, was asked if she did this in honor of her host country. He said, "It's a great idea, but we can't claim any credit for it." But then, both British and U.S. flags are red, white and blue, so maybe she was dressing for Britannia . . .

One of Ohio's most enduring political figures, Frank J. Lausche, celebrated his 90th birthday Sunday. The event brought together two bitter political rivals, Ohio Gov. Richard Celeste and former Gov. James A. Rhodes. Rhodes had previously announced he would run against Celeste, describing his administration as the "most corrupt and inept" in the state's history, but in honor of the evening they shook hands. Lausche's political career stretched from 1932 to 1969. He served three terms as a judge in Cleveland, two terms as Cleveland's mayor, five terms as governor and two terms in the U.S. Senate, making him virtually a one-man political dynasty . . .

Actress Lynda Carter has been named National Crusade chairman for the American Cancer Society. She will be making public appearances and public service announcements . . .