Inspector Knacker of the Yard had searched a house in County Antrim on Monday afternoon and arrested a boy on suspicion of Computer Misuse Act offences.
He has been taken into custody at Antrim police station and is being questioned by detectives from the Police Service of Northern Ireland.
A police statement said this was a joint investigation involving the Police Service of Northern Ireland (PSNI), and detectives from the Metropolitan Police Cyber Crime Unit (MPCCU).
The TalkTalk website was hit by a “significant and sustained cyber-attack” last week and banking details and personal information could have been taken.
The company said it did not know how much of its customer information had been encrypted. Over the weekend, TalkTalk’s chief executive claimed the attack was “smaller” than originally thought.
Dido Harding said any credit card details taken would have been partial and the information may not have been enough to withdraw money “on its own”.
Card details accessed were incomplete – with many numbers appearing as an x – and “not usable” for financial transactions, it added.
The hack sparked calls for the government to do more to force companies to encrypt data. Culture minister Ed Vaizey told the House of Commons the government was not against compulsory encryption for firms holding customer data.
Clearly he has not talked to his boss. David Cameron believes that encryption is the tool of choice of terrorists and companies should be forbidden from using it.