PlayStation Blog has announced that the 10% discount code for purchases on PSN is live as of today as a compensation over the PSN outage over Christmas. Remember that the discount stacks on top of other offers, so be smart about how you use it. There are some restrictions, for instance you cannot use the discount when renewing PS Plus and you cannot use the code when purchasing stuff on your PS Vita (you'll have to make the purchase via the webshop). Full details in the blog post.
The code you'll need to use for the United Kingdom is T6R3KB529K, for Ireland it's C4EFQJ4KFB. You'll find more codes on PlayStation Blog.