It looks like a Rory O'Brien that I used to have, judging by the nice detailing on the dropouts and where the stays meet at the seat post cluster.
I found an O'Brien on the net:
Where the chainstays meet the dropouts it's different detailing to yours - mine resembled the ones in this picture. Also the fork is different to this one, but the lugs are the same.
It looks like a Rory O'Brien that I used to have, judging by the nice detailing on the dropouts and where the stays meet at the seat post cluster.
I found an O'Brien on the net:
Where the chainstays meet the dropouts it's different detailing to yours - mine resembled the ones in this picture. Also the fork is different to this one, but the lugs are the same.