In Irish Murtaugh is derived from Ó Muircheartaigh, but nowadays the prefix is rarely attached to the name. The form O'Moriarty and indeed occasionally MacMoriarty is found in old records. The main sept, which is of the same stock as the O'Donoghues and the O'Mahonys, has always been associated with Co. Kerry. Their original territory lay on both sides of Castlemaine Harbour, but after the Anglo-Norman invasion their influence was reduced by the uprise of the Fitzgeralds, in spite of an early alliance by marriage in about AD 1210.

Ó Muircheartaigh is also the name of a sept located near Kells, Co. Meath. Like other minor septs in that part of the country they were soon submerged after the Anglo-Norman invasion; but their descendants have survived, as Murtaugh, and this name is fairly numerous in the midlands.

The Murtaghs/Murtaughs of Ulster and North Connacht, where the name is also found, are in many cases of Scottish origin - in Scotland the form Murdoch is more usual.

In some cases descent may be from Clann Muircheartaigh a sept of the O'Conors and Síl Muiredaig (Silmurray).

The Murtaugh/Murtaghs of Co. Mayo have essentially 3 potential sources of origin:

1) Murdochs from Scotland, most likely those from around Strathclyde and Galloway, by way of Ulster through the transplantation period of the 17th century,

2) Clann Muircheartaigh (of Silmurray) descendants, and

3) Descent from someone else named Muircheartaigh, such as Muircheartaigh MachEaghra (Mac Hara/Gara). This not being an uncommon name.

In regard to 2 and 3, above, it should be noted that the largest concentration of Murtagh's in Mayo are located between the original territories of these 3 groups (O'Hara, O'Gara, and Silmurray.)

MacMurdy - Co. Cavan

MacMordie, Murdoch - Ulster

MacMurty, MacMurtry - Antrim

MacBrearty - Co. Donegal

Mortagh, Murtagh, Murtaugh - Co. Mayo (used synonymously with Mortimer)

Murtha, Murtagh - Co. Meath

(O/Mac)Moriarty - Kerry