Longford went into the match bottom of the Premier Division as manager Tony Cousins made four changes to his starting line-up, including giving a league debut to 17-year-old defender Tristan Noack-Hofmann.
The hosts snatched the lead on 26 minutes as David O’Sullivan powered to the net.
Derry should have been level at the interval, though, as Rory Patterson had a weak 35th-minute penalty saved by Paul Skinner while Dean Jarvis rattled the crossbar.
McClean, younger brother of West Brom and Ireland winger James, broke Longford’s resolve on 73 minutes with a cracking 35-yard strike to the corner.
The result sees Derry, now seven games unbeaten, slip two points behind leaders Dundalk while the point lifts Longford two places off the foot of the table.
Meanwhile, there is just no stopping Limerick as they beat Cabinteely 3-0 at Markets Field to surge ten points clear at the top of the First Division.
Garbhan Coughlan put Limerick in front on 35 minutes and they doubled their advantage seven minutes after the restart when Cabinteely skipper Kevin Knight turned Shane Tracy’s corner into his net.
The young Dublin amateurs had defender Lee Mitchell sent off after conceding a 70th minute penalty. Tracy converted from the spot to complete a fine night for Martin Russell’s runaway leaders.
Cobh Ramblers came from 2-0 down to deprive Athlone Town of a first win of the season at St Colman’s Park. The point was enough to lift Alan Mathews’ midlanders off the foot of the table on goal difference, however, as Cobh remain third.
Midfielder Val Feeney volleyed home David Cassidy’s corner to put Athlone ahead on 49 minutes.
Two minutes later, Emeka Onwubiko raced through to double the lead.
Skipper Shane O’Connor led Ramblers’ revival when curling a shot to the net on 77 minutes. Seven minutes later Cobh were level - Rob Lehane slotting home from the penalty spot following a handball in the area.
In tonight’s EA Sport Cup, second round action, Cork City entertain Waterford United in an all Munster clash at Turner’s Cross. Holders St Patrick’s Athletic travel to last season’s double winners Dundalk while Shelbourne host Bohemians and Finn Harps face a trip to Galway United.