If 3/1 accurately reflects the probability of a second referendum before 2019 then the real odds of it not happening is 1/3. The fact they offer 1/5 is merely them scalping you. Conversely if 1/5 is the real price for no referendum then you should be offered 5/1 to bet on it to happen. It's also possible they are taking a cut of both prices, but how you can argue their prices are not an indicator of probabilities is beyond me, the skew may not allow for a 100% accuracy but they are an indicator nonetheless.
The raisin d'etre of bookies is to scalp you, m8. Therefore their "predictions" are unreliable. That's the point we're both making I think.
Both prices above can't be accurate indicators of probability. One or both are therefore inaccurate. It's not possible to say which. So to say that a price of 2/1 translates to an actual probability of 33% is flawed logic from the start.
There's a tennis tournament going on at the moment. Two possible outcomes in each match. Head over to Paddy Power's website and have a look at the odds for the winner on each match. I guarantee you that you won't find a single pairing that follows your logic.
The way people make money off the bookies is to identify instances where the actual probability of a given outcome is greater than the "probability" represented by the bookies' odds.
http://www.peoplesrepublicofcork.com/forums/showthread.php?t=232156
http://www.peoplesrepublicofcork.com/forums/showthread.php?t=236912