It depends on the monthly repayments of your credit union loan. The longer the term, the lower the repayments, and hence the lower the negative impact on your mortgage application. Of course, normally it is terrible advice to recommend paying a loan over a longer period (higher total cost of credit in the end), but if the mortgage is the priority, then it makes sense, and you can accelerate the repayments manually yourself.
The other point to note is that the credit union loan repayments will affect your application to different degrees with every bank.
Best Regards,
Dave Curry, Irish Mortgage Corporation
https://ie.linkedin.com/in/davecurryirl (LinkedIn profile and client testimonials)