The 'November Man' star's adopted daughter Charlotte, 41, died after a three-year battle with ovarian cancer on June 28, and his good friend Lisa Paulsen says the actor, 60, always made sure she had access to the best care, latest clinical trials and treatments throughout her illness as he hoped he could help her get better.
Lisa, who is the president of the Entertainment Industry Foundation, told People.com: "Pierce was willing to pick up the phone and call anyone, go anywhere, take her anywhere, do anything for her at anytime. He was desperate."
Pierce adopted Charlotte and her brother Christopher, the children of his first wife Cassandra following the death of their father Dermot Harris in 1986.
He and Cassandra went on to have a son Sean in 1983 before her death from ovarian cancer in late 1991.
Lisa also revealed that she believes despite losing his daughter and first wife to the disease, the Irish actor has done a lot for cancer research worldwide.
She said: "After Cassie passed away, Pierce became a fierce champion and ambassador to all things related to women's cancer research.
"I told Pierce, 'Despite the fact that Charlotte didn't end up benefiting from long-term new ovarian cancer therapies, so many things that you have done have led to so many discoveries.' He's a remarkable and loving father, and Charlotte was such a trooper."
Pierce has been married to Keely Shaye Smith since 2001 and the couple have two children, Dylan, 16 and Paris, 12.