McDonald

Richard:

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Likely they lived in Westport Demesne. There are actually two locations with this name. One near the town of Westport in Oughaval civil parish. The other was in Kilmeena civil parish north of Oughaval parish.

Roger McDonnell

You can find more info about the two townlands named Westport Demesne (in Irish, Diméin Chathair na Mart) at these links:

In Kilmeena civil parish:  https://www.townlands.ie/mayo/burrishoole/kilmeena/kilmeena/westport-de…

In Oughaval/Aughaval civil parish:  https://www.townlands.ie/mayo/murrisk/oughaval/westport-urban/westport-…

You can find the Catholc parish records for Aughaval which are avaible online at this link:  https://registers.nli.ie/parishes/1043

You can find the Catholc parish records for Kilmeena which are avaible online at this link (though they are less comprehensive than those for Aughaval) : https://registers.nli.ie/parishes/1146

Keep in mind that, in Ireland, the surname McDonald is a variant form of the name McDonnell, and you might see either in parish registers or in offical records.  According to MacLysaght’s "Surnames of Ireland", the McDonnell surname is of three separate origins, two of them related to one another.  There was a gallowglass (in Irish, gallóglach, or “foreign fighter” = mercenary) family living in the Glens of Antrim (many people in Ireland have ancestry in such gallowglass families), and a western Connacht sept (or tribe), living principally in what is now Mayo, which is related to the first group.  The third family lived further south in Ireland, and was unrelated to the first two.

In his "Lore of Ireland", Ó hÓgáin explains that the McDonnell's in Antrim were related to the Scottish MacDonald clan.  The McDonnell’s came from Scotland to settle in the Glens of Antrim in the 13th century, from where, by the 15th century, they controlled all of North Antrim.  If you've been to Ireland, the big ruined Dunluce Castle which is a popular tourist site near the Giants Causeway on the North Coast was a McDonnell family castle.  A large number of those McDonnell's were brought from Antrim to what is now Mayo in the late 14th century, to assist the ruling family of that area, the Anglo-Norman Burke family.  Some also live across the border in neighboring counties (the county boundaries were a later creation).

The surname in Irish is Mac Domhnaill, which means “son/descendant of Donal”, but it is spelled MacDhòmhnaill in Scottish Gaelic (with the same meaning).  The "d" at the end of MacDonald is a later addition to the anglicized version of the name.  MacDonald is more common in Scotland (and among Scots settlers in Ireland, especially in Ulster), but McDonnell and McDonald were both used in Ireland as anglicized forms of the surname, sometimes within the same family in various records.

As Catherine, the younger daughter was born in England after civil registrations of births began there (almost 30 years before Ireland), you should be able to find out her mother's maiden surname. However, you can't be sure that she had the same mother as the older children, especially given the high death rate in Ireland in that decade.

 I'm puzzled by the 2 little girls recorded with occupation servant. What work could a 2-year-old do?

By the way, you put 1951 census in your opening sentence instead of 1851. You can make corrections and amendments to your posts.