An international text costs 25c so maybe you sent one and that explains the drop from €10 to €9.75?
If you use an iPhone then such a text could have been sent automatically as part of the Apple iPhone activation process which send a UK SMS.
Did you just join 48 and do you use an iPhone?
If that doesn't explain it then you must have used some other service that was charged to your credit balance.
I don't understand the second problem.
You can't top up by less than €1.
Check your My History page in your My 48 online account to check what payments/purchases are listed.
If necessary contact 48 support for an explanation.
http://www.48months.ie/help#ask-a-48-agentHope this helps.