If you hover the mouse over the link, what URL is displayed? If it isn't something like 'http://www.tesco.ie/shoppingvouchers', then it's spam. You did the right thing Slim, if in any doubt, do not click on links. That alone can be enough to expose you to certain exploits.
For reference, in a URL, the two elements before the first single / are the most important. If it's
www.tesco.ie, then, that's a valid Tesco domain. If it's [broken link removed], then that isn't tesco, and will bring you to wherever the owners of the asite domain want to bring you.