Oh dear, sounds like you're off to a good start with these two! Chances are, they have indeed lived in HMOs (Houses of Multiple Occupancy) prior to this tenancy and don't realise the fire regulations differ for non-HMOs.
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 Belfast landlord.
You can download this guide by clicking this link.
Hopefully, a couple of smoke alarms and an updated tenancy agreement will smooth things over with your new tenants, and the rest of their time in your property will be stress-free, but to avoid confusion with any future vacancies, you can consult my guide to managing tenancy applications.
There's some good advice in there for managing tenant expectations and clarifying rights and responsibilities before keys are handed over. Hope this proves useful too!
Are you having any issues with tenants or properties in your rental portfolio, or simply have a question about the Belfast property market? Drop me a line to firstname.lastname@example.org and I will do my best to solve your head-scratchers!