Deaths in Islandeady - Castlebar

I can’t account for what happened to three children of Michael Moran and Rose Needham who lived in the Castlebar - Islandeady area of Mayo in the first half of the 1800s. All three children were born in the 1840s (Ann in 1840 https://registers.nli.ie/pages/vtls000632042_008 , Pat in 1844 https://registers.nli.ie/pages/vtls000632042_054, and John in 1848 https://registers.nli.ie/pages/vtls000632042_098). Michael and Rose left for Macclesfield, Cheshire in the 1850s (and then later on to Australia) with four of their other five children (they all appear in the 1861 England Census) and their other child had already left for Australia by the mid-50s. Rose’s Australian death certificate lists all her children but says that 5 (that’s 10 in total, not 8) had died before 1890. So, the death certificate showed an extra 2 children who had died but my guess is this was likely to be a clerical error at the time of death.

My assumption is that Ann, Pat and John died as children during the 1840s in the Castlebar/ Islandeady area, but I can’t find any records of their deaths. It’s been pointed out to me that RC deaths were not recorded and that I could try the CoI death records. I’m not sure how to do that.

Where would they have been buried? Is there a list of burial places/ church graveyards in the Castlebar/ Islandeady area that I could search?

Any help in solving this problem would be greatly appreciated.