This decision rests with you and you have no obligation to share your CF diagnosis with your employer.
Your main workplace protections come from the Employment Equality Acts and these laws make it illegal for employers to discriminate against you because of a disability.
If you do choose to share your CF with your employer it will enable them to support you in the best way that they can. It will also enable them to make ‘reasonable measures’ to help you do your job which for example include flexible working arrangements or time off for clinic appointments.
Ultimately, this is your decision, if you would like more information on working with a disability please check out Employment and disability