At Antrim Crown Court, sitting in Belfast, a defence lawyer asked for the charges to be put again to Paul Greatbanks, 48, and he pleaded guilty to murdering Patrick Harkin on 20 February 2011.
Greatbanks, whose address was given as Maghaberry Prison, also admitted trying to murder Letterkenny man Paul Mythen, 39, in the same incident.
No details were given in court, but previous courts have heard that at the time of the killing Greatbanks lived in the flat below where Mr Harkin, 49, was found dead at Bayview Terrace in the city, just yards from Strand Road PSNI Station.
The married father of one, who tragically died from the serious head injuries he sustained, was originally from the Creggan area of the city and belonged to a large well-known family while Mr Mythen, who also sustained head injuries, is well-known in the north west as a film-maker and musician.
Following the guilty pleas on Monday, Mr Justice Horner told Greatbanks there was only one sentence permitted by law, namely life imprisonment.
The judge told Greatbanks he would fix the minimum period he must spend behind bars before he is even considered for release next month once pre-sentence probation and victim impact reports have been compiled.