Frenchman Jean-Francois Lucquin (71) and Sweden's Anders Forsbrand (68) shared fourth place at 1-under-par 287.
Italy's Massimo Florioli, who held the 54-hole lead after the third round was completed Sunday morning, struggled to a 4-over 76 and tied for sixth with Adam Crawford and Philip Walton. The group came in at even-par 288.
Mednick trailed Florioli by a stroke when the final round teed off and quickly grabbed a piece of the lead. Mednick ran home a four-foot birdie putt at the second hole to share the lead at 4-under par.
Mednick, an American-born Swede, took the outright lead when he two-putted from 20 feet at the par-5 fourth. He dropped a shot at the fifth when he landed in a left bunker but Florioli also lost ground with a bogey at the same hole, two groups behind Mednick.
The Swede birdied the ninth and when Florioli bogeyed eight, Mednick was in charge of the tournament. Mednick collected one bogey on the back and birdied three holes, including the last two, to walk away with the title.
'It has been a fantastic week and it was nice to finish with that birdie on the last,' said Mednick, whose best finish this season was a tie for 16th at last week's Wales Open. 'When I first came here and drove up the driveway I said this place looks fantastic. I love links courses and links golf and I am really glad we played on a links course this week. Ballyliffin is right up there with the best of them.'
The 35-year-old earned his tour card for 2002 at Qualifying School but that will not be a concern next year. This victory gives him a valuable one-year exemption on the European Tour.
'This win means the world to me,' said Mednick. 'I was on the borderline for not getting into tournaments in the future and I haven't had many starts this year, but now I can plan my season exactly the way I want to the rest of the year and next year as well.'
Coltart had only one top-10 finish this season, a tie for sixth at the Open de France. He fashioned a bogey-free round of 5-under 67 to share second place.
'I am starting to hole some putts and that makes a huge difference,' said Coltart, who established a new course record at the Glashedy Links of Ballyliffin Golf Club with a 66 on Friday. 'I'm delighted with the way I have putted this week. It gives me a good confidence boost going into the next few weeks.'
For Rocca, it was a chance to earn his first title since he won the inaugural North West of Ireland Open at Galway Bay in 1999. He could only muster an even-par 72 Sunday and would have a solo second but missed a four-foot par save at the last.
'It would have been nice to have won it again' said Rocca, who beat a young Tiger Woods in the Sunday singles at the 1997 Ryder Cup. 'If I had been 4-under par after the front nine I might have a chance. Tried hard but didn't hole anything.'
Joakim Rask of Sweden finished alone in ninth at two-over par, followed by Michele Reale of Italy, who took 10th at plus-four.
Full-field scores from North West of Ireland Open