You will have to complete a mortgage application form listing your current loans. You will also need to sign a declaration stating the information continued in the application form is true.
If you want to lie on the application form, make sure there are no direct debits going from your bank account to the credit union. If the credit union is on the Irish Credit Bureau then the bank will see it.
Honesty is the best policy, if they refuse you because of the loan then maybe you should not be taking on extra debt for the home improvements.