This is a common problem on safranes. You can have the heater blower on full blast but you'll get very little air out. In the UK at least, the safrane does not have a pollen filter so do not even consider this (and that applies to the 2.5 with all the luxury stuff).
You'll find the problem is probably the tubing in the top of the dash behind the airbag on the passenger side and behind the speedo unit.
On some Safranes they're made of plastic, others are simply cardboard rolled up and glued together. After 16 or so years you can imagine the condition this will be in.
You should be able to get the general idea of how to fix it from this european version. Obviously the sides of the car are inverted for the UK / Irl version. These images are borrowed from renaultsafrane.nl:
Before removing the airbag make sure you've disconnected the battery for at least half an hour as it is electrically triggered.
Also note that the airbags in the seats are IMPACT triggered and thus can trigger without the battery being connected.
Ignore the red circle in this picture
Once fixed you should also find your demister works a lot better too
