where you are charging 1.15 for each additional kg over 15, the amount =that needs to be subtracted from the Northern Ireland Over 15kg 20Supplement rule is:
1.15 * 15 = 17.25
the rule should be created as:
Subtotal of column 'TotalWeight' 'times' '1.15' 'minus' '17.25'
to be sure what the total weight in the cart is, add the following code to your page:
<?php echo WA_eCart_DisplayWeight($eCart1, $eCart1->TotalColumn("TotalWeight")); ?>