As a general guide, you can let someone go within the first year as they are not covered by the Unfair Dismissals Act.
However, there are also anti-discrimination rules. You can't discriminate against someone who is disabled. They could claim that their illness is a disability . While they would be unlikely to succeed, they could tie you up in the tribunals and courts for a while.
You should issue an employment contract to them. Make sure that it specifies that sick leave is unpaid.
Then if their attendance does not improve terminate them.
Make sure that there is no probation period in the contract.
Make sure that the notice period is short.
Brendan