The directors are ultimately responsible for ensuring the company complies with the law, including the Companies Acts.
The company secretary, if one is appointed (see question 3), shares legal responsibility for Companies Act compliance with the directors.
The directors will usually delegate day-to-day responsibility for administrative tasks under the Companies Acts to any secretary they appoint, although the directors remain jointly responsible with the secretary for any default.
The directors may also give the secretary responsibility for legal compliance in other areas of law, and for other administrative tasks (see question 1). The company secretary's Companies Acts responsibilities apply automatically, but any other responsibilities will depend upon the secretary's contract of employment.