There are 2 civil parishes in Co. Roscommon called "Killukin". You have joined the wrong parish!
Lisdaly townland belongs to the parish of Killukin (Boyle). The RC Parish records you need to search under are Croghan aka Killukin aka Killukin-Killummod aka Ballinameen aka Aughrim (and for post 1865 BMDs they registered under BOYLE or CARRICK-ON-SHANNON).
So the Ballinameen / Boyle result above is a very good lead for this couple. Absolutley Lisdaly territory.
As for the Farrell history in Lisdaly...
1749 Elphin Census recorded a Garrett & Rose Farrell (RC) Cottiers there. Most likely, taking you 2 generations back! That no Farrell or Mulvey were listed on the Tithe Applotment records here in 1842 does not mean they were not residing there. They may have been part of a group / family holding.
Other relatives you can follow up on:
On Lisdaly's eastern boundary, in Ballinvilla, Martin Farrell & Cath Dowd of Ballinvilla were baptising a child in 1889 (again, they had no land-holding here, probably cottagers).
However, I think your people may have hailed from Drumerr (two townlands, east of Lisdaly) as both Mulveys and Farrells lived there for a long time:
1842 [Tithe Applotments] see Ml. Farrell who had 9 Irish acres (subject to tithes, most of Drumer was bog and did not qualify).
1857 [Griffiths Valuation] See Pat Farrell at farm 4a Drumerr.
1865 CRO Pat Farrell & Mary McGarry; also Peter Mulvey (labourer) & Catherine Beirne
1902 Pat Farrell & Margaret Harrington
You also have a cluster of Farrells appearing on Tithe Records of 1835 in Ardlavagh (Drumerr's northern boundary) and Legvoy aka Gardenstown (adjascent). You can be sure they all stem from the same ancestor originally. Ardlavagh & Legvoy were Farrell AND Mulvey strongholds. So, I'd recommend you dig deep in both of those townlands if you wish to extend your tree further. Overall, there wasn't an abundance of Farrells in the parish so you should be able to pin it down without too much confusion.
See also: FARRELL & DOWD died 1792 in Killukin Old Cemetery.