An attic conversion is generally a good way of adding space and adding value to your property, because you're working with an existing structure instead of having to build from scratch. When you consider that average build costs were around €2,500/sqm in 2006, then the quote received works out at around €500/sqm, which seems reasonable but it will depend on the quality of finish proposed. Ask to see previous examples of work at this price and for references from previous clients.
Generally, if you are making changes at roof level, like adding dormer windows front or back, then you will have to apply for planning permission. Also, one of the key issues for attic conversions is the height achievable in the finished room - this is a building regulation requirement. If the room does not comply with the Regs, then it cannot be marketed as a habitable room and will not add value to the property. As you are still at the early stages, it is worth checking on the height issue.
For a summary of planning & building reg requirements for converting an attic, follow this link and there's a 'Loft Conversion' leaflet in the Publications box on the right hand side:
http://www.environ.ie/en/DevelopmentandHousing/BuildingStandards/