Bruce,
Killevan Church of Ireland has records starting in 1811 so that might be early enough to catch Thomas & Ellen’s marriage if they married there, and of any children born between 1812 and 1819. Tradition was to marry in the bride’s church after which she’d attend her husband’s, so its possible that marriage and baptisms could be in different churches. Some years for Killeevan Church of Ireland are on the rootsireland site (subscription).
Newbliss Presbyterian has no baptisms before 1856 so if they were Presbyterian you may struggle to find anything.
The tithe applotment records (which list those with land) for the parish in 1825 list 6 Williamson households: John in Cornawall, an un-named one in Drumhillagh, Alexander in Drumlina, William in Golanduff , John in Killigowan & Anthony in Lecklevery.
The tithe applotment records list 1 Laird farm – John in Golanduff. So that gives you Williamson & Laird in that same townland. There were only 4 houses in Golanduff in 1861. It was quite common to marry the girl next door, so if you get no other clues it’d be worth bearing in mind. Both families had gone by the time of Griffiths Valuation in 1861. No Laird households anywhere in Monaghan by the time of the 1901 census. Just the one house and family in Golanduff in the 1901:
http://www.census.nationalarchives.ie/pages/1901/Monaghan/Newbliss/Gola…
I looked for births, deaths & marriages for Oliver but found none.
There are 106 Williamsons in Monaghan in the 1901 census. 32 were Church of Ireland and 74 Presbyterian so twice as many Presbyterians as COI. (All are likely to be descendants of families that arrived in the 1600s).
