Leading 1-0 from the first leg in Riga, John McDonnell's Saints were nonetheless made to work hard to break down the young Latvians.

With Keith Fahey orchestrating midfield, Mark Quigley was Saints' chief foil but he wasted two good openings before the 24th minute; his first shot from a Fahey cross deflected over while he then volleyed well off target after Derek O'Brien and Glen Fitzpatrick worked the chance.

A set-piece finally broke Olimps resolve on 40 minutes.

Nathan Junior was booked for a foul on Des Byrne.

But further punishment arrived from Fahey's superb delivery with Harris powering a downward header to the corner of the net.

A trip by substitute Alens Snelnikovs on Fitzpatrick brought the second goal from a 70th minute penalty which Quigley despatched - sending keeper Kaspars Ikstens the wrong way - for his 15th goal of the season.