Hi in 2013 my visa to visit the USA was rejected on the grounds of that there were insufficient funds for my travel but this has changed will this affect my chances of getting a student visa to Ireland? I lost the refusal letter and am thinking of answering no previous visa refusals on the online form. Any advice from those who were in a similar situation?
Never lie in a visa application, to a visa official and to an immigration officer when arriving in a country.
If you know your application in the USA was rejected answer 'yes' and give as many details as you can, (if you know year and reason, write those) and mention you do not have the letter (anymore.)
The visa officials can check things in a shared database or they might ignore the information as not relevant.
But if they catch you on a lie they will take action, most likely a ban, which will influence your future travels much more than your current rejection.
Visa officials are much less likely to ignore a previous ban than a previous rejection.
I do not have links for this, I have learned this reading a lot of questions and answers on this site.