You can BUT the only add ons for 5 euro (the "bonus") are 100 text or 1GB.
If you want to just buy add ons then top up by 10 euro (or less). Any higher amount (15 or over) will activate the 4G plan which will take 15 off your balance leaving 5 "bonus" which expires along with the plan after 28 days.
If you're still on the 3G plan (where you keep the top up) you are probably better off staying on it.
I really wish I've read this thread earlier. Same happened to me. I saw an ad about Tesco 4G and switched over (because, frankly, the mobile Internet was SO slow on Tesco 3G, so I was hoping this would make it better). But they SO CONVENIENTLY forgot to mention that, with the new plan, I will be loosing 15 euro credit on every topup. When I first noticed it, I thought there was a mistake. But no, that was totally how they planned it. It is unbelievable that a company can pull a scam like that and get away with it.
Key summary of terms:
•There is no ability to change back to a legacy plan once signed up.
•Top Up by €15 every 28 days
•€15 will be deducted from your account in order to activate the Offer
•Credit of €5 will be added for you to spend on out of bundle services
•No carryover of Offer benefits after the 28 days including the credit
•Existing credit will be transferred across to your new plan
•4G availability is dependent on device capability, coverage and plan
Tesco Mobile Ireland Customer Care