WASHINGTON--President Reagan asked Health and Human Services Secretary Margaret Heckler yesterday to give up her Cabinet post for the ambassadorship to Ireland, the White House announced.

Heckler asked for and was given a few days to think about it, presidential spokesman Larry Speakes said. Heckler met with Reagan alone in the Oval Office for 50 minutes yesterday afternoon to try to talk him out of reassigning her.

Heckler, after leaving the White House, ignored reporters' attempts to question her. A spokeswoman said the secretary would have no immediate comment on the session with Reagan.

Although Speakes repeatedly characterized the job offer as a promotion, it would mean a pay cut of about $15,000 a year and the loss of a department with 145,000 employees and, as she was fond of saying, the largest budget in the world with the exception of the entire U.S. and Soviet budgets--$330 billion. "That's a lovely position--for someone else," Heckler said of the ambassadorship last month.