APY stands for Annual Percentage Yield, a calculation done by the financial institution to make simple comparisons of account value after 1 year between competing accounts. The APY includes the effects of compound interest regardless of the rate of interest, so the simple answer is: no, your return is only your principle multiplied by the APY after a year.
Credit Unions are more member participatory than a bank, so the name "share" implies that you own a share of the credit union and it's future. It's possible that the CU could elect to pay a dividend on top of your interest rate. Since you have the option to credit the dividend to the share certificate account or another place, it seems that interest would be paid on the dividends left in the SC on the compounding schedule at the contracted rate.
You would have to look at the terms of the account to verify precisely when the dividend is payed, whether it's a value above and beyond the interest, and how it is compounded into our account.