Allergy to onion is very rare. It is also rarely a cause of allergic contact dermatitis and urticaria.
Onion is in the Alliaceae plant family, other plants in this family are chives, leeks and garlic. If your allergy is severe these plants may also cause allergic reactions.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Onion.php
The allergy resources page for onion covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with onion.
There is also regularly updated links to useful websites specifically for onion allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.