“Serious is not a word to use in decorating,” is one of Los Angeles interior designer Kathryn M. Ireland’s favorite axioms. Maybe that’s why in her 30-year career she’s had a lot of clients with a sense of humor, including Steve Martin and Drew Barrymore.

She shares some of her Hollywood projects and her own homes in the new book “A Life In Design” (Simon & Schuster), out Oct. 24, which celebrates her fearless approach to mixing colors, global styles and fabrics.

“I think everyone in the book is still in their houses,” says Ireland, who moved from London to L.A. in the ’80s, and has lent her style to Spanish Colonial houses, ranches and farmhouses up and down the California coast, in the Hamptons, and in the British and French countrysides.

She got her start when she turned her filmmaker husband’s editing studio in Santa Monica into a shop selling decorative accessories, which became an instant hit. Ireland was also a design writer, and when Steve Martin saw her article in House & Garden about actors Amanda Pays and Corbin Bernsen’s renovated home, he wanted something similar. That’s when she got her first professional job as an interior designer for his cottage.

Taking into account Martin’s colorful modern art collection, she chose fabrics with more subtle seafoam green and cream hues and interesting textures as a contrast; designed simple yet luxurious curtains to hang in the windows and French doors, and created relaxed seating areas conducive to conversation or an impromptu banjo session.

“Every job I’ve done has been a huge learning curve. Leading with art and antiques, I learned that from Steve,” she says.

Ireland had between Labor Day and Thanksgiving to transform cosmetics giant Victoria Jackson’s ranch house in Ojai, California. She stripped floors; remodeled bathrooms, and added Moroccan and Mexican tiles, Spanish light fixtures and European furnishings, with lots of unexpected touches, like curtains made from vintage Mexican serapes and red floral-shaped light fixtures from Blanchard Collective in the master bedroom.

“I like the unexpected, but not kitsch — I don’t mind the odd garden gnome, I suppose, but if you do small bits of unexpectedness, it’s amusing,” says Ireland.

For a British actor’s farmhouse in the Cotswolds, she did a full-scale renovation but stayed true to the spirit of the place that once belonged to Sir Mark Palmer, the hippie baronet and former page to his godmother Queen Elizabeth II, and his wife, astrologer Catherine Tennant.

She wallpapered the master bedroom in her Marrakech Natural in Teal, accompanied by a French armchair upholstered in Pampas Teal by Andrew Martin. The armoire was from Lorfords in Tetbury, and the kente cloth bedspread was made by an Ashanti tribe in Ghana. Ireland’s love of Ghanaian fabrics goes back to the ’80s when she first traveled to the country to make a documentary film. Those fabrics became the inspiration for her own fabric collections, featured in the book.

“I always look at my rooms and say it’s kind of like the United Nations. Everyone gets on in my rooms. You’ve got Bali, Ghana, France, all living in the same room. I believe that you don’t want one thing to shout out to you in the room. You can have quite a lot of things shouting at you, but they’ve got to shout together. It’s harmony, I suppose,” she says. “That was the beginning of my textile collecting, apart from the fact that I did win the sewing prize at age 7 for my patchwork quilt.”

She also sewed her own clothes, and still does. “I’m doing kind of like Vivienne Westwood meets Saint Laurent for my son’s wedding,” Ireland says. “I came out of Fashion. I worked at Feathers, and actually assisted designing a line of clothes for Arabella Pollen,” she remembers. “I took the collection to New York and I put on Fashion shows, including one at Studio 54 at the end of its heyday, and one at Xenon. Fashion for the most part is color, and home is an extension of Fashion. As a young girl, all you think about is fabulous clothes, because houses seem so unobtainable.”

But now? “Decorating is therapy. I just love doing it for myself and for other people. I suppose it’s my form of relaxation.”