The coupon interest will be taxed, as you mentioned, as case III at your marginal rate. The issuer will be obliged to withhold standard rate income tax at 20% for which a credit will be available to recover the WHT. You will need to declare income as it accrues to you on a normal form 11 and make a annual tax return
You may also have a PRSI liability of 3% if the income exceeds €3,174. Basically, when that threshold is reached and your a PAYE contributor under class A, you become a self employed cont under class S.
Here is an extract from tax summary clarifing same:
"There is no liability to PRSI where reckonable income is below €3,174".
Good luck