Here is a website that has some price comparisons for home heating oil - http://www.cheapestoil.co.uk/Heating-Oil-NI.aspx. It also gives where they deliver so that can be helpful for narrowing down your choices. They give the date that they checked the prices and many of them are very current. Here are some other websites that you can find helpful - http://www.emooil.com/, http://www.maxoldirect.com/, http://www.boilerjuice.com/heating-oil-prices-northern-ireland.php, http://www.cheaperoil.com/qazwsxedc/files/company_Profile.asp (offers quote tools right on pages, some are direct company quotes).
Please let me know if this information has been helpful or if you need more.