There's sure to be some electrical cables right above the ceiling. Pipework is also possible. It all depends on the construction of your dwelling and how the services have been installed.
Always best to fully survey your home so you know how the services work and where they run. At the very least, trace all cabling and pipework in your work area. It may seem tedious when all you want to do is install some simple fixtures, but it's not worth just drilling away and hoping for the best. Otherwise you could:
Damage wiring, potentially starting a fire.
Puncture a water pipe, potentially causing a flood.
Penetrate a gas pipe, potentially causing an explosion.
Do you have access above the ceiling? A loft or attic hatch?
Also, how are you planning to secure the brackets? If fasteners are going into the ceiling you need to locate them into joists. It's possible to fit lightweight fixtures directly to ceiling plaster but a curtain and rod have some substance, and will be pulled.