According to the Longman Dictionary of Common Errors,

"The noise of the traffic outside all day annoys me." is not good English and "The noise of the traffic outside all day irritates me." is better.

This dictionary explains that "Something unpleasant that happens repeatedly or continuously over a long period of time tends to irritate or frustrate people, especially because they know that they can do nothing to stop it."

I still do not understand the difference between "annoy" and "irritate." Could anyone explain the difference with some sentence examples?
One difference in usage that comes to mind is that annoy isn't usually used when the action one finds unpleasant isn't deliberate. Therefore we say:

It irritates the eyes to look at the sun.

But: His behaviour annoys me.

In my opinion irritates would also be possible in the last sentence, though.
Another example: As a diplomat, he had to be able to cope with annoying remarks.

CB
The meanings of these two are pretty similar. However, while both of them can be emotions, only "irritate" can can refer to a physical sensation. Your skin can be "irritated" by a certain fabric. The example above points out that you eye could be "irritated" by looking at the sun. In these physical cases you could not use "annoy".

Teasing apart a difference between then when they apply to emotions seems more difficult. I'd say that "irritation" might imply a longer period of exposure than "annoyance". This would also tie in the emotional meaning with the physical one as usually physically irritating things often take a while to become noticeable.

I wouldn't want to suggest this as a hard and fast rule though, as there will be times when the two words are truly synonymous.
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I should think the speaker ought to choose between them according to whether he is annoyed or irritated by the situation. Two people may react differently to a given source of annoyance/irritation. Emotion: smile
A small fly trying to land on my eyeball tends to be both, though I doubt he intends to annoy me.
AvangiI should think the speaker ought to choose between them according to whether he is annoyed or irritated by the situation.
Avangi, this is a marvelouspiece of advice! Emotion: big smile - Have one on me! Emotion: beer

CB
I don't see much difference, except that "irritate" seems to suggest there could be something that has been bothering you for a while... and maybe "to annoy" is more common in the most informal registers. But I can't think of a way to distinguish them clearly. And I don't want to. That'll save me an unnecessary headache! Emotion: big smile
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I sometimes think we're annoyed on the conscious level and irritated on the unconscious level. Not that we don't become aware of an irritation - but the annoyance plays with our mind. This can relate to CB's point about the human element (deliberateness), although we can be extremely annoyed by someone's "mindless" actions.
I agree with you.

I think "irritates" is a stronger emotional exxpression than "annoys". After long being exposed to it, one feels irritated. Being "annoyed" there is still a possibility of the condition being reversed. Eg: After all these years and the tabloids are still "speculating" over the scandal of ......... It is very irritating. Your mouth is full, kindly stop talking and spitting food all over. It is very annoying.

I hope I am right.
I have just joined as a member. Please forgive my typo-error on the word exxpression. The correct word is expression.
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