Muckians are to be found today in Ireland, England, Scotland, Wales, Canada, the USA, Australia, New Zealand and maybe more. There is a family website at www.muckian.net.
Apparently our O Mochaidhean ancestors in the barony of Cremorne, County Monaghan, Ireland generally anglicized their names to (O)Muckian, (O)Muckyan, Muckean, Muckeen, Mackeen or Mackean under the penal legislation which banned the use of Irish names. At the time of the Great Hunger, 1845-49, many Muckians emigrated, and the spellings of the surname in the records varied as officials wrote down what they thought they had heard. So far, I have found Mackey, McIein, Muckin, Macken, Mucker, Muchian, MacKain and Muckion, most of which became Muckian again by the end of the nineteenth century as literacy spread. In the Twentieth Century, many Muckians changed their names to Mackian, MacKian or even MacKain. In some of the Irish Civil Registration records, individuals are recorded as both Muckian and Mackian.
All these changes make for confusion as many of the names chosen, such as Macken, Mackin, Mackeen, Mackian, Mackean and Mackey are all valid names of other families! The hope is that Genetic Genealogy will help us gradually unravel the story and find out whether the rumour that we Muckians are of the line of Niall of the Nine Hostages is founded in truth! There is a Muckian DNA Project at FamilyTreeDNA  and the one member tested so far has the haplotype R1b1c7 M222+ (Ysearch  record C4PW7)
The former Herald of Ireland, Edward MacLysaght, says in his book "The Surnames of Ireland", that in medieval times the Muckian (O Mochaidhean) sept held a leading position in the barony of Cremorne in Co Monaghan, and the Rev Patrick Woulfe writes that it is "the name of an Oriel family who were anciently lords of ... the barony of Cremorne in Co Monaghan".
In O'Dugan's Topographical Poems (The Topographical Poems of John O'Dubhagain and Giolla Na Naomh O'Duidhrin, edited by John O'Donovan, 1862), there is a reference to "O'Mochoidhein, king of Mughdhorna", and this is possibly the family earlier referred to as Uí Machainéin in the Annals of Ulster. The Annals of the Four Masters mention Oissine ua Machainén, lord of Mughdhorna, in an entry for 997. More digging to be done here!