Hello Karen, I am looking at unanswered queries and Callaghan and the many variants are a tricky search, I also have Callaghan in my ancestors but the surname Sheils or Shealds in you case is a bit more rare, however I cannot find an exact match for you and there is always the possiblity the particualr parish does not have records for the years mentioned.
I found no marriage to suit but in Upper Creggan, Armagh an unknow Calahan married a Catherine Sheils, the reason is the page of the register is torm apparently. The marriage took place in the brides parish normally, the date is 15th January 1831, Armagh is now in Northern Ireland. No parents names given and witnesses are a James Sheils and Thomas Calahan.
I can find no baptism to suit your John but the parish above them Lower Creggan does not have baptism records for this period and the appear to start in 1845. I did check for other children to no avail. .
You can see the actual registers here but only search by event and date in month and year and then read, can be a hard read too.
https://registers.nli.ie/ My Callaghan were from Cavan and a DNA match told me the name came in the late 1700s from Cork and they were cattle and horse dealers with mission to buy or rent land, no idea if true but there is a lot of them in the north Cavan and Fermanagh area, first came to Lislea he was told by a distant cousin in the USA, the name is more found in the Cork area on the south coast.
For Sheils you can search free here for name distribution https://www.johngrenham.com/surnames/ after that a subscription is required.
You could also check the Tithe Applottments for the principal name of the person paying tax, had to have an acre or more. See
http://titheapplotmentbooks.nationalarchives.ie/search/tab/home.jsp
Good Luck
#Pat