It is not compulsory to fit smoke alarms or fire detection equipment to a rented property unless it is classed as an HMO. However, I'd advise it's good practice to fit alarms to the same standard as a new property - it protects you from challenge in case of a fire.
What I'd say is to make sure that you enshrine it all very clearly in your tenancy agreement.
State what you have provided (a good quality battery operated smoke alarm on each floor should suffice) and make a provision in the tenancy agreement that the tenant is liable for testing the smoke alarm periodically, as well as for ensuring that the batteries are replaced as needed, and the alarms themselves are kept free from obstruction.
If you do have an HMO, then yes, you must fit smoke alarms and fire detection equipment.
The Northern Ireland Fire Service and the Housing Executive have produced an excellent guide to fire safety, which goes into great detail on what should be done to protect occupants from fire in this type of property.
HMOs vary wildly - from your typical shared student house, through to supported living facilities for people with additional needs and mobility limitations. So it's good to know exactly what's expected of you as a landlord.
A couple of smoke alarms and an up to date tenancy agreement will smooth things over with your tenants, and the rest of their time in your property will hopefully be stress-free.