Graham Burke, who fired Rovers into an early lead, incurred his second needless red card of the season 12 minutes from the end just as the fifth-placed home side were chasing a lead they badly required to catch Bray Wanderers and Derry City.
With little provocation, the former Aston Villa trainee tried to kick out at Michael Barker which referee Rob Hennessy punished with a straight dismissal.
It was the type of momentary madness that has proved counter-productive during a season Rovers has promised much without delivering.
Despite the Saints starting brightly by creating a couple of chances for JJ Lunney, they were caught napping by Rovers for the opener on 18 minutes.
There seemed little danger when a pass was played into Gary Shaw’s feet 25 yards from goal but his flick-on dissected the Saints defence and Graham Burke cut in from the left to tuck the ball beneath Conor O’Malley.
For all their possession, the Saints were still prone to another defensive mistake and Rovers were almost handed another gift nine minutes before the break.
Instead of saving a free-kick directed straight at him by Graham Burke, O’Malley parried the effort to Ryan Connolly six yards out whose rebound was denied by the goalkeeper.
It wasn’t just Liam Buckley’s side being generous as a defensive mix-up by Connolly and Tomer Chencinski handed Fagan a clear chance to lob the goalkeeper from 20 yards which he failed to take. His scuffed shot trickled horribly wide.
Fagan wouldn’t be so profligate when presented with another chance ten minutes after the restart. Conan Byrne made a foray into the Rovers half, allowing Lunney thread a pass for Fagan to control and then smash a shot into the top corner from 20 yards.
Then followed Burke’s indiscretion, triggering a period of panic in the Rovers defence. Having been the better side for the most part, Stephen Bradley’s side spent the final stages on the back foot and were fortunate in the end to escape with a point.
Had Conan Byrne or Lunney been potent in the box when supplied with clearcut openings, the mid-season review at Tallaght Stadium may be more unkind on a boss still learning in his first full season at the helm.
With another suspension looming and Paul Corry ruled out with another injury, Rovers need to find a consistency in their game to not alone challenge Dundalk for second spot but also consolidate a top-four spot and ensure European football.
Chencinski; Madden, Lopes, Webster, L Byrne; Finn, Connolly; Miele (Boyd 84), Burke, Clarke (Meenan 66); Shaw (O’Connor 80).
O’Malley; Barker, Peers, Feely, Bermingham; Cregg, Desmond; C Byrne, Markey (K Byrne 84), Lunney (A O’Hanlon 89); Fagan.