What happens if beer ferments below the optimum temperature?

First, take a reading of specific gravity to determine where you were in the fermentation before the temperature dropped. This will tell you how much further you need to go (meaning how many days to expect further fermentation to go at proper temperatures). Simply bringing the temperature into the correct range will be enough to re-energize the yeast. You don't have to stir the wort if this is inconvenient, since this might introduce bacteria and aeration you don't need.

Bring the temperature up to at least 70F, but ~75F as a maximum, and wait a day or two for signs of re-fermentation (air lock bubbling, for example). It will take at least a day for the carboy to react to the change in ambient temperature. An Irish Ale ferments best at slightly lower temperatures in your range, so you can let the temp drop to the "upper 60s" once the fermentation has re-started. Then just proceed as if nothing happened.

40 degrees is quite a bit lower than the bottom range for your yeast. I'd expect that they've gone pretty much inactive. But don't worry! All you need to do to reactivate them is to warm your brew back up to the optimal temperature and provide some gentle agitation. Be careful not to splash! As fermentation has already stated, you don't want to add any oxygen into your beer.

And yes, you may need to add a few days (equal to the number of days spent at 40 degrees) to allow for complete fermentation.

Additionally, it is possible that your yeast finished anyway before it got too cold. You could take a gravity reading to see if it meshes with your expected FG.