What could be the cause for frequent throat congestion and irritation?

Allergies, other. Post nasal drip from allergies and sinusitis is a common cause of throat irritation. Acid reflux (which is a cause of heartburn) can also irritate the throat. (see dr. Melder's comment below).
Many possibilities. Allergy can do it, low grade infection can do it. One very common cause, though, is what you eat and drink. The throat, especially around the voice box, is very sensitive to acid. People who drink a lot of coffee or tea, or who like spicy foods, often complain of swelling and irritation with "post nasal drip." this is due to actual inflammation of the throat tissue leading to swelling and mucous.