Couple of ways to do it:
Increase ISO, shoot at something like 1/60th and shoot wide open - you should get a decent exposure that way. You might have horrid noise though. Expose so you get the right balance between blowing the highlights and the rest of the model...
The other way is to use a speed light on low power and then use a long(ish) shutter speed to 'burn' in the ambient, which will include the fairy lights. The speedlight will probably have to be on a lowish power - what you want to avoid is suing a high f/stop to control the flash because that will mean the ambient will take longer to burn in, meaning the risk of camera shake. It's all about knowing what the meter for and how to expose the shot...
This was shot the latter way:
Beer Can 3 by
Pat MacInnes, on Flickr
Low power flash on the bottle (hence the f/2 aperture) and a long shutter speed (1/6th sec) to burn in the fairy lights in the background