The board of Franciscan Hospital for Children in Boston, one of the country's largest hospitals for severely disabled children, has fired longtime CEO Paul J. DellaRocco, citing financial irregularities. Board chairman Robert Needham said in written comments to the Globe that DellaRocco was "inappropriately submitting and documenting expenses. This type of behavior is clearly at odds with both our policies and our nonprofit mission," he said. "We therefore took proactive steps to remove him from the position." Needham declined to provide further details, but a public relations professional hired by the board said the hospital will not pay DellaRocco severance. He was fired Thursday. Reached at his home, DellaRocco disputed findings that he billed the hospital for expenses he should not have. "I have been 18 years on the job; I understand it's sometimes important to have a change in leadership," said DellaRocco, who indicated that he would fight for the severance pay he believes he is entitled to under his contract.