MacMahon was the most common family name in County Clare at the turn of the century 1900. McMahon is one of the best-known and most distinguished names in Ireland. In Irish MacMathghamha or in modern spelling MacMahuna it is said to be derived from the Irish word for a bear.

It is borne by two quite distinct septs. The more important of the two belongs to Co. Clare. These descend from Mahon son of Murtagh Mor O Brien, King of Ireland (d. 1119) and the last inaugurated Chief of the name fell at the Battle of Kinsale in 1602. Their territory was Corcabaskin in West Clare. The Ulster Sept of McMahon in the thirteenth century became Lords of Oriel on the decline of the Carrolls. It is associated chiefly with Co. Monaghan. As is usually the case with the old Gaelic families their present day representatives in Ireland are still to be found chiefly in their original territories in this case Clare and Monaghan.

Because of its importance and because it was numerically strongest in County Clare at 1900 let us examine its Arms.

Looking at the McMahon arms above we see that it consists of the three main components, the crest, shield and motto. The crest is formed by a dexter arm in armour embowed proper garnished or holding in hand a sword both proper pommel and hilt or.
The shield is formed by argent three lions passant reguardant in pale gules armed and langued azure.
The motto, 'Sic nos sic sacra tuemur' means 'Thus, we keep holy things.'

We notice that the colour of the shield is argent (silver), depicted as white, indicating peace and sincerity, and the lions are in azure, or blue, symbol of loyalty and truth.

The motto of the Clare MacMahons, 'Thus, we keep holy things', clearly shows the association the MacMahons had as the custodians of the church properties of the O'Briens.