Wondering if it is my 2nd great grandmother in 1821 census

Statutory registration didn't start in Ireland till 1864 for births and 1845 for non RC marriages. Prior to that you need to rely on church records. Not all churches have records for those earlier years and of those that do, not all are on-line.

It's possible that the lady in the 1821 census was your relative but I don't see any way of knowing for certain. Kinawley is about 20 miles from Enniskillen. If they married in Enniskillen, then it's possible your ancestor lived there. You haven't said what denomination the family was but I'd guess Church of Ireland. Unfortunately Enniskillen Church of Ireland's early records were lost in the 1922 fire in Dublin (ironically sent there for safe keeping) and only some fragments remain now. So it;s possible there is no record of the marriage or of the children's baptisms to find. The remaining fragments for Enniskillen cathedral are not on-line so far as I am aware. They are held in PRONI in Belfast and a personal visit is required to view them.

Thank you for your detailed reply. As you say, I may never know.

John Appleby was in a Scottish regiment of foot, so Scotlandspeople.gov.uk kindly gave me the marriage certificate when I asked them. By 1841 census they were settled in Minsteracres agricultural complex in Northumberland, England. It's documented as RC, & seemed to have connections with Yealand in Lancashire where he was born, RC. But on none of these records do I find a reference to their parents unfortunately!

My father did say that he thought his family background was "lapsed catholic" further back but  certainly he had not been brought up in that faith.

Even the marriage certificate does not name their parents, which I found unusual

 A couple of witnesses are named. But I haven't been able to trace them. They may have been in the regiment as well I suppose.

The earlier children who were born in Ireland are likely to have been RC as well- I'm not sure where to look for their births. I put their details in my original post.

So it's a story about the United Kingdom really!  I was surprised how many regiments there were in those days as I was researching.

Many thanks for your interest.

The marriage record has witnesses Mary Ann Healey & Mark Roberts

Here are two baptisms from RootsIreland. The first one could be your ancestor. The second one, Thomas, is definitely yours—his father is with the 75^th regiment.  It is possible the couple had a son  Thomas who died, and they named a later son, born in Northumberland, Thomas.  Or perhaps this Thomas is your direct ancestor.

Name:   Elizabeth Appleby          

Baptism:              12-Jul-1827

Church Of Ireland

Parish/District:   Oughaval Church of Ireland parish

Co. Mayo

Father:  John Appleby      Mother:               Mary

Name:   Thomas Appleby            

Date of Birth:     05-Oct-1828                     

Parish/District:   Birr

Co. Offaly

Father:  John Appleby                 Mother:    Mary

Occupation:        75 Regt.                             

Such good information that I never knew about.

I can add them to wikitree as births- your theory about earlier children not surviving is probablycorrect!

My great grandfather Thomas died in 1930 aged approx 85 ( I don't have his exact DOB)

And another Elizabeth was 2 in 1851 census!

I'll add the earlier births to wikitree. And record the possibility that they died in infancy. John must have been moved around with the regiment for them to be born in different counties. It's improving my knowledge of Ireland's geography as well!

Thank you very, very much