Your tenant is entitled to peaceful occupation of the property while living in the property. This means the tenant can invite guests to stay or have someone move in with him without your consent. If, however, there is a term in your tenancy agreement which forbids subletting, the tenant cannot receive any rental income from people living with him.
Subletting a room can help a tenant to afford a property, particularly if the tenant has been affected by a reduction in the amount of benefits s/he receives. You should not unreasonably refuse consent if the tenant wishes to sublet a room. Refusing your consent may mean you lose a reliable tenant.
You can only stop your tenant from subletting if there is a clause preventing this in the tenancy agreement. If your tenant is lawfully subletting, the other person in the property is not a full tenant but will be protected by the rights of the actual tenant while the actual tenant remains in the property. This means you cannot force the person sub letting to leave, while they have the tenant's consent to remain. They can only be evicted through legal action.
If your property is a HMO, subletting could present a serious problem for you. There are very strict rules which govern how many people can legally live in a HMO. The number of residents in a property is restricted by the size of the rooms in the proeprty.
If your HMO is overcrowded, the Housing Executive may take legal action against you and you may have to pay a substantial fine. You should take any reports of overcrowding seriously and investigate these. Talk to your tenants if you suspect that they are subletting rooms in the property and make sure they understand the impact of their actions.
If you can show that the overcrowding is the fault of the tenant and you have not been receiving any additional rental income, you may not be convicted of an offence.