Eric Wilson shot roofer Dan Smith, 24, eight times in the face and body.
Dubliner Wilson, 27, whose nickname is Lucky, faces up to 25 years behind bars.
Mr Smith, himself on the run from UK police over a 2007 attempted murder, had tried to stop Wilson “touching up” a woman in a Costa del Sol bar, a court heard.
Wilson raced by motorbike to his rented home, grabbed a 9mm pistol and returned to shoot Mr Smith, known as Tall Dan, on the bar terrace.
Police raided the killer’s home two days later in June 2010 and found the bike, hand grenades, explosives and ammunition.
A jury in Malaga was shown documents which suggested Wilson had killed 11 people in Ireland.
He is due to be sentenced in the next two weeks.