Hi Mindi
working backwards here is the census of 1911 and 1901, default 1911, other partial census available also but very little.
See http://www.census.nationalarchives.ie/search/
A free site but you must sign in has protestant civil marriages from 1845 and BMDs from 1864, all births have entry available but only certain deaths and marriages can be viewed in full from ledger, church records on this site are mainly Dublin; Kerry and some Cork. https://civilrecords.irishgenealogy.ie/churchrecords/civil-search.jsp
A land survey was done between 1848 and 1864 I think, only owner and lessor given but you can open separate page and see map, from 1st icon take plot number and then fiddle with map icons to find number in townland and you can draw bar to top right of map icon to left to give modern map and a lot of zooming is required to establish location, see http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameS… If searching maps is your thing you can view Irish maps here https://www.osi.ie/
Between 1824 and 1838 tithes were imposed and just names of those who had over an acre of land are here, just name and townland given, http://titheapplotmentbooks.nationalarchives.ie/search/tab/home.jsp
In 1796 a list of flax growers was made and I see some few Kelso on it, remember this is just those growing flax,http://www.failteromhat.com/flax1796.php
In the 1600s there was a Hearth tax called money rolls, (hearth as in fireplace I think) but only some available and not Down but they are here there are no Kelsos in the Money Rolls but in searching this site I see a trade directory with one Kelso in Dublin, a saddle maker in Bridge Street. http://www.failteromhat.com/kelly1915all.php The money rolls are on this site too, look at home page under Census/Directories.
The land change over or Plantation of Ulster took place between 1640 and 1680 and is called the Down Survey, nothing to do with Down as in County Town but in the putting down of chains to measure the land, think it was biggest ever done at teh time, no Kelso names but here is link http://downsurvey.tcd.ie/index.html and click on Historical GIS for search facility.
I am adding the catholic marriage registers but they are a difficult read and I had assumed they were Protestants, this assuming business in not good research, you need a month and year to use for the best as name search is not available.
http://registers.nli.ie/
Protestant Church of Ireland records are being placed online here, all these records are on subscription sites so if you have access to one these should be available. https://www.ireland.anglican.org/about/rcb-library/online-parish-records
These are all the primary sources but different areas are creating local sites and often a location plus history search will bring them up, or sometimes you may find a Face Book page for the area with active participants.
Happy Hunting and revert if any furrther queries, in about 2 months this site will allow you to place information on the site directly to the parish of interest and that may pick up others looking at the same parish.
Pat