Graham Dwyer has lodged an appeal against his conviction for the murder of Elaine O'Hara.
Dwyer was sentenced to life in prison last month for the murder in August 2012.
The Courts Service confirmed this morning it had received papers appealing his conviction.
A jury found Dwyer guilty of murdering Ms O'Hara in a unanimous verdict and he was given the mandatory life sentence last month.
Mr Justice Tony Hunt said it was a chilling and premeditated murder and we may be thankful this dangerous man was now out of the way.
Dwyer had 21 days to lodge an appeal. That time ran out today.
The Courts Service confirmed this morning it has received notification that he is appealing his conviction.
He is understood to have posted papers from prison on Friday.
The grounds of his appeal are not yet known but it is likely they will include issues raised by his lawyers during the trial.
These include the lack of a cause of death for Ms O'Hara, his interviews with gardaí, evidence relating to mobile phones, videos shown to the jury and the judge's demeanour during the trial.