How do I handle it - an employee on sick leave is due to attend for occupational health assessment and I expect that they may not be fit for all of their duties.  What are the key issues to watch out for from a legal perspective?

The Employment Equality Acts prohibit discrimination on nine protected grounds, one of which is disability.  An employer must not subject an employee to less favourable treatment on grounds of a disability.  The legislation also provides that an employer is not obliged to recruit, train, retain or promote an employee who is not fully competent to undertake the duties required of their role.  This provision is subject to a very important caveat.