Manchester City will have to beat Steaua Bucuresti across two legs in order to reach the Champions League group stage.
Pep Guardiola's men, who finished fourth in the Premier League last season under former manager Manuel Pellegrini, have been drawn against the Romanian side for the playoff round.
It is a favourable tie for City in terms of opposition as they could have come up against tougher teams in Roma and AS Monaco, but they face a lengthy trip to Romania.
Brendan Rodgers and his Celtic team also have some long travel ahead of them as they have been drawn against Israeli club Hapoel Beer-Sheva.
The Scottish giants sealed a place in the playoff round after defeating Astana 3-2 on aggregate in the qualifiers.
Dundalk, who play in the League of Ireland Premier Division, have been drawn against Legia Warsaw, who are the reigning champions in the Polish top flight.
Elsewhere, Portuguese side Porto will go head to head with Italian giants Roma, while La Liga's Villarreal will take on French club Monaco in the playoffs.
The first legs will be played on August 16 and 17, with the return legs a week later on August 23 and 24.
Champions League playoff draw:
Ludogorets vs. Viktoria Plzen
Celtic vs. Hapoel Beer-Sheva
FC Copenhagen vs. APOEL FC
Dundalk vs. Legia Warsaw
Dinamo Zagreb vs. Salzburg
Steaua Bucuresti vs. Manchester City
Porto vs. Roma
Ajax vs. FC Rostov
Young Boys vs. Borussia Monchengladbach
Villarreal vs. Monaco
