The council directive 2000/35/EC referred to above shows that you are somewhat protected, although I'm not sure how this plays out from a legislative perspective in Ireland. Regardless of this directive, if your contract with the customers say 90 days, then that's the one that applies.
If you are dealing with a large company, presumably they are working to a standard 30 or 45 day payment commitment, and for some reason their accounts dept. are not fulfilling this. If this is true, you have a few options:
1) build up a relationship with somebody in accounts (the more junior the better) and find out if they do an organised payment for all suppliers on a certain day. Figure out what day your invoice is normally paid, and make sure it is submitted (and dated!) about 5-7 days before that date. It might take a couple of months to figure this out and get it right, but after a while it should work;
2) work out if it is better sending in one invoice, or lots of small ones. Sometimes it is easier to chase one big one.
3) establish who the most influential "business owner" is within the company. An occasional internal visit/call from them to their accounts might be received better by a busy accounts dept.
Let us know how you get on!
Bob