I came across this theory and thought is was interesting, looking at it from an Irish Perspective
People will be unwilling to publicly express their opinion if they believe they are in the minority. They will also be more vocal if they believe they are a part of the majority. Thus, the more marginalized you become, the less you speak and so spiral into a fully marginal position. This works because we fear social rejection. and that when a person appears to be rejected, others will back away from them, fearing being rejected because they associate with the rejected person. It also makes marginalization a powerful way of eliminating political and social competition. Public opinion is the “attitudes or behaviors one must express in public if one is not to isolate oneself, in areas of controversy or change; public opinions are those attitudes one can express without running the danger of isolating oneself.
The crucial points of the theory are
People have a fear of being rejected by those in their social environment, which is called “fear of isolation.”
People are constantly observing the behaviors of those around them, and seeing which gain approval and disapproval from society.
People unconsciously issue their own threats of isolation by showing signals of approval or disapproval.
Threats of isolation are avoided by a person’s tendency to refrain from making a statement about something they think might attract objections.
People are more willing to publicly state things that they believe will be accepted positively.
The spiral effect begins because when people who are seen as representing majority opinion, often authority figures, speak out confidently. The opposition feels a greater sense of fear of isolation and is further convinced to remain silent, since they perceive themselves to be in the minority. The feelings continue to grow in either direction exponentially.
A strong moral component is necessary for the issue to activate the spiral.
If there is a social consensus, the spiral will not be activated. There must be two opposing forces.
The mass media has a strong influence on this process.
Fear and threat of isolation are subconscious processes.
The spiral of silence only “holds a sway” over the public for a limited time.
If a topic activates the spiral of silence, this means that the issue is a great threat to social cohesion.
The critics of this theory most often claim that individuals have different influences that affect whether they speak out or not. Critics believe that there are three potential influences besides the fear of isolation that could cause the spiral of silence. 
1.) Many researchers have studied whether influences of close social networks can influence a person’s willingness to speak. It is found that people have the same opinion of their social networks. However, it is not clear as to whether our fear of isolation is greater among acquaintances and stranger than in our close networks. “In the smaller context of friends and family, we may feel safe expressing our opinions since we already perceive their opinions as similar to our ours.” 
2.) Scholars have also questioned whether personal characteristics have an influence on whether a person will willingly speak out. “Naturally, if one has a positive self-concept and lacks a sense of shame, that person will speak out regardless of how she or he perceives the climate of public opinion.” 
3.) Another influence critics give for people choosing not to speak out against public opinion is culture. The culture that a person lives in greatly affects their willingness to speak out. “Not every culture holds freedom of speech in as high regard as the United States, and in some cultures, open expression of ideas is forbidden. “  Some cultures are more individualistic, which would support more of an individual’s own opinion, while collectivist cultures support the overall groups opinion and needs. Cultural factors could also be gender. “Perhaps another explanation for why individuals do not express minority opinions can be made: that women’s perception of language, not public opinion, forces them to remain quiet.” 
Thinking back to the time of the Bank crisis in Ireland, and how taxpayers learnt that they would be paying that for the greed and mistakes of the elite few, the prevailing attitude was that there wasn’t anything we could do about it, mass media reinforced the mantra by politicians that the bank bailout was “the only show in town ” even though there were other options available.
Some people spoke and acted out, the ‘call for a general election’ campaign, was one, that had support, but it never gathered momentum. People felt that there was nothing they could do to stop austerity measures being enforced on them, and on political forums and elsewere the protests were laughed at, and mocked. How many people would have liked to show their support but were silenced into doing so, by the ‘majority’? Eygpt demonstrated that results are possible from protesting.
Then there was Eirigi, who occupied some bank buildings, they were outwardly ridiculed for doing so, because their political stance didn’t sit comfortably with people, but I wonder how many people inwardly supported what they were doing, but were afraid to voice this support, because Eirigi weren’t popular judging by mainstream opinion ?
Another example was the first Gulf War, and I’m sure the findings could easily be also applied to the second Iraq war
This study analyzes actual and perceived support for the Persian Gulf War in the United States. Data were collected from 292 residents of New Castle County, Delaware, during the 1991 Gulf War. Results show that support for the war was not the strong consensus reported in mainstream media. In fact, 53.1 percent of the respondents fell within the neutral, disagree, or strongly disagree ranges of a support for the war scale. Only 6.6 percent of the respondents were in the strong support range. However, responses were significantly higher on an item measuring perceived support for the war. Consistent with Noelle-Neumann’s spiral of silence theory, perceived public support for the war was a significant predictor of support for the war even after 13 variables were controlled. The alternative explanation that subjects were ‘projecting’ their own perceptions onto the public, is discounted by the finding that liberals, moderates and conservatives did not differ in their perceptions of public support.
The ‘Occupy’ movement will be an interesting one to watch. Will the mainstream media and ‘majority opinion’ succeed in limiting support for it, or will people, encouraged by events in Egypt and else were, speak out and get behind it?