"I had always been a tomboy - I still am, at heart"

In this quote, Maureen O'Hara is expressing that she has constantly been a tomboy, which element of her character still remains true to her. Being a tomboy indicates that she has actually constantly been more interested in activities and pastimes that are typically connected with kids, rather than those usually related to girls. This could consist of things like playing sports, climbing trees, or getting dirty. By saying that she is still a gamine at heart, O'Hara is suggesting that even as she has actually aged, her core character and interests have actually not changed. She is proud of her gamine nature and accepts it as a part of who she is.