I used my local Credit Union a lot growing up. My CU is Drogheda Credit Union, and they've been taking over other smaller CU's in recent times, so they seem to be doing relatively well and would be perhaps one of the larger ones. As far as I know, most (all?) CU's follow the same general rules.
When it comes to lending, if you have savings, your savings are secured against the loan. Sometimes they won't require the whole lot if you mention it to them, but by default they are entirely secured to the loan.
My CU has two loan products, as a result. A standard loan (8.9%) and what they call a "Special Savers" loan (5.9%). You qualify for the Special rate, if you are borrowing an amount of money that is less than your savings. (ie; if you have 5k saved, and get a loan of 2k).
If you have 5k saved, and borrow 10k, you pay 8.9% on the entire amount, however, once the balance of the loan is lesser than your shares, your interest rate swaps to the lower rate of 5.9%.
So you absolutely are paying interest on your own money. However, they do stress these days that a loan application is decided on your ability to repay, and not your shares, so i presume you can talk them out of securing your shares, however, as i say above, the 'default' method seems to be to secure your shares.
There is something to take into consideration here though in relation to your shares: If you have shares with the CU, you will get some 'free money' if you die as they have insurance in place that'll match your savings, up to €7,700. Of course, for someone in their 20's buying a first car, this is likely not a great concern, but worth a mention, nonetheless.