Lighting is always a factor, always! Photography is the "drawing of light" after all. Here are a few key points to remember if you can't afford movie-style light set ups...

Small & dim light sources (especially fairy lights etc) are not particularly strong, and especially in darkness/at night when there are no other light sources available. Dark rooms = dark shots. 

To combat the darkness (as most photographers will assume you do not want a set of black photos) the photographer will have to use a low aperture, such as 1.4. This will make your pictures very creamy, blurry, with only one or maybe two people in focus given that they are "on the same level". Do not expect everyone in the room, or even on a table, to be sharp. Personally, I like using a low aperture! 
Another method of brightening a photo is using a high ISO. This will make the pictures grainy. Again, some people don't mind/kind of like grain on photos. To combat this in situations, flash can be used.

If you don't want flashed-out pictures but want to keep the cute, fairy light esque light set up, you can either grow to like creamy, grainy pictures, or ask for lights to be turned up at key points (e.g speeches, first dance). Alternatively, you can always rent some continuous lighting and aim it at the top table etc. 

The entire marquee was decorated with fairy lights, enough light was provided because of the white backdrop all around which gave the light chance to bounce back.

With this wedding, they had fairy lights that supplied even less light than the one above - but because the venue room had a large window at the front and down the sides, enough light was reflected back on to the couple and guests.

This venue was particularly dark so for this part of the wedding i asked for lights to be turned up just for the first dance. 

These kinds of venues are just heaven to me - large windows = so much light!! 

But if your venue isn't like this than alterations can be made!! Extra lighting, lights being turned up etc. You can always make it work for you.

Just something to think about when searching for your perfect venue!