The Shield is: Argent three lions passant reguardant in pale gules armed and lanqued azure.
The Crest is: A dexter arm in armour embowed ppr. garnished or, holding in the hand a sword both ppr. pommel and hilt gold.
The Motto is: "Sic Nos Sic Sacra Tuemur", 'Thus We Guard Our Sacred Rights'.
Variant spellings for McMahon are: MacMahon, Mahon, Maghan, Mann, Maughan
Gaelic spelling: "MacMathghamha".

The name itself is said to come from the Irish word for 'bear'.

Two great families of the MacMahon name found in Ireland. The first are the MacMahons of Thomand (Clare). The second are the MacMahons of the old territory of Oriel in the 13th century.

The MacMahons of Co. Clare descend from the O'Briens, the posterity of Brian Boru, and are of separate descent from the MacMahons of Monaghan of the Clan Colla. The Clare MacMahon family is found in the List of the Septs of Thomond, A.D. 1317. The lands of Corcabaskin, Moyarta, and Clonderlaw were included in their territories.

The tomb of the family of MacMahons of Clonderlaw is mentioned as being at the monastery at Ennis in 1642. Liskaloge anciently (before 1680) belonged to the MacMahons. A MacMahon castle at Carrigaholt in Clare was once a stronghold of the family.

The O'Donnells or MacDonnells were the original chiefs of the West Corabaskin district, they were sunk under the MacMahons, the descendants of Brian Boroimhe who came to assume the land of Corca Baskinn, controlling the baronies of Moyarta and Clonderlaw in Clare down to the reign of Elizabeth.

In Kilconry Parish in 1580, a castle in the island of Feenish belonged to Bryan MacMahon (surnamed na-Foraire).

Tiege MacMahon of Clonderalaw owned a castle in 1580 in the townland of Caher-da-con (Cahercon). Nothing of the castle remains today. This was in the Kilfiddane parish.

The 1890 birth index shows "Mahon" in Dublin and Galway, with McMahon in Clare, Monaghan, Limerick, and Dublin.

The Irish Family "McMahon (12)" is recorded in the Upper & Lower Tulla, 17 in Clanderlaw, 19 in Islands, 21 in Moyarta, 27 in the Upper & Lower Bunratty, 8 in Ibrickan, and again 9 in Corcomroe, all the above are districts in Clare in the census of 1659.

Mahon is also found as a principal name in the 1659 census of Ireland, in Co. Clare. Most readers will realize that this name is likely McMahon, or MacMahon, with the prefix dropped.

The family remains numerous in Co. Clare into modern times. Some are found settling in Chicago and New York City.

Arms for the name are found on plates 26, 57, and 153 in the Irish Book or Arms.

Source: Families of County Clare
Michael C. O'Laughin
1997 Irish Genealogical Foundation