Sounds like the usual iPhone activation issue.
For some reason known only to Apple, iPhones will only work out of the box on a specific list of networks.
For other networks activation is needed before they will work.
This process can require a UK SMS to be sent which on 48 costs 25c.
This SMS gets sent automatically by iOS online or during a sync with iTunes (depending on the version of iOS used).
So try putting some cash credit on your account to cover the cost of this text and try again.
It may not activate immediately so give it a bit of time.
If it doesn't happen automatically then try syncing with iTunes.
If it still doesn't work then post back with more info or contact 48 support:
http://www.48months.ie/help#ask-a-48-agent
Note that this issue is down to Apple and how they manage "known" networks and is not specific to 48 (e.g. also happens on Tesco etc.).
Hope this helps.
