Assuming you're talking about bulk diesel cost delivered to Ireland, its notoriously difficult to get hold of ... you can get pricing info off Platts (for a fee, approx 12k per annum IIRC).
I also seem to recall a supplier in the Midlands which issued a weekly summary for much less, about 100 per month.
If you know what you paid bulk in a given week, you can use pump prices to calculate an estimate ... adjusting for duty etc. Of course that's no use if you're looking to check/estimate the margin at the pumps ...