The couple have been seen together on a few occasions over the last month, but according to Niall's One Direction bandmate Louis Tomlinson's sister, they are now officially dating.
Writing on Ask.fm, 13-year-old Félicité 'Fizzy' Tomlinson said: ''I haven't asked him, but when I was at 'X Factor' [USA, earlier this week] Alberto from security said Niall's girlfriend Barbara was here.
''Liam [Payne] said everyone has girlfriends here tonight except Harry [Styles] so yeah they're dating.''
Earlier this month Niall, 20, introduced Hungarian-born beauty Barbara, also 20, to his bandmates - which also include Zayn Malik - at a party in London.
A source said: ''The rest of the group were properly introduced to Barbara, though most of them met her briefly before. This time, though, it was as Niall's date.''
The couple were also spotted cosying up on a cinema date to see 'The Hunger Games: Catching Fire'.
An eye-witness said: ''They were laughing and joking together throughout the film, and both were dressed casually. You could tell they were comfortable in each other's company.''
Before dating Barbara, Niall was linked to 'X Factor' UK contestant Tamera Foster and British reality TV star Louise Thompson.
The album is the first in history to chart without selling a single copy.
The musician passed away while performing with his band OHM in LA.
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