A Sub-Lease is a Lease from the tenant to a sub-tenant.   A tenant can in some circumstances itself then lease the premises to a third party by granting a Sub-lease.  If the premises are Sub-leased, there will be two leases in operation- the Lease from the landlord to the tenant and then the Lease from the tenant to the sub-tenant. A Sub-lease should make sure that the Sub-lessee has to comply with the provisions of the first Lease to ensure that there is no breach of that Lease

Two important aspects of a Sub-Lease are that it can only be granted with the consent of the landlord and it must be for a shorter term than the Lease between the landlord and the tenant.
Generally, the same principles apply to a Sub-lease as apply to any lease except that legislation like the Conveyancing Act, 1919 affords rights to the Sub-lessee beyond those given by the Common Law.
Sub Lease - Lease Hub