Are opinion polls reliable?

The Republic of Ireland has just had a referendum to amend their constitution. There were two amendments proposed. One was to change references to "marriage" and replace it with "durable partnership". The constitution also explicitly says that women and mothers do valuable work in the home and that the state should support them - the second referendum question was about removing this.

Before the referendum campaign started, opinion polls were as follows (Yes = agree to changes, No = keep constitution as it is):

Issue 1:
Yes: 53% No: 15%

Issue 2:
Yes: 60% No: 12%

The results of the referendum has just come in:

Yes: 26%

No: 74%

The polls were massively out. Massively! I don't think we've seen a polling failure like this before.

There are two explanations for this.

The first is that people don't pay much attention to politics in day-to-day life, so when they respond to opinion polls, they glibly choose the option that "sounds good". But once the campaign starts and they have to make a hard decision, they pay close attention and ask difficult questions. And often opt for the status quo as a result.

The second explanation is down to the large number of "Don't knows" in the opinion polls. These are people shy about admitting they don't like the fashionable changes being proposed. But in the privacy of the polling booth, they're free to vote No.

Across the Irish Sea, this referendum result will cheer up UK Tories immensely. Because opinion polls show them on 20% with about 20% "don't knows".

It's unfashionable to admit you want to vote Tory at the moment. But unemployment is 3.8%, inflation is coming down and no-one knows what Labour stand for. So in the privacy of the polling booth...