1. English and Irish (Norman), and French: habitation name from any of various places in Normandy and N(orthern) France, originally named with the L(atin) personal name Lúcius [See Lúcio] + the local suffix -ácum.

2. English: from the medieval female given name Lucie [L Lúcia, a fem. form of Lúcius]. The name was borne by a young Sicilian maiden and an aged Roman widow, both martyred under Diocletian and venerated as saints.

3. Irish: Anglicised form of Gael(ic) Ó Luasaigh, an altered form of Mac Cluasaigh "son of Cluasach" a byname originally denoting someone with large ears [from cluas ear.]

Vars.: Lucy. (Of 2 only): Eng.: Luce [the normal medieval vernacular form]; Lucia (Latinised).

Cogns. (of 2): Fr.: Luce. It.: Lucia.

Dims. (of 2): Fr.: Lucet(te).

Metr. (from 2): It.: De Lucia.

A powerful Anglo-Norman family called Lucy took their name from Lucé in Orne, Normandy. Richard de Lucy (died 1179) was a baron and chief justiciary of England. He fought in Normandy and commanded the castle of Falaise, returning to England in 1140. His son Godfrey de Lucy (d.1204) became bishop of Winchester in 1189.

Over the centuries, there have been many changes in the spellings used in Irish, (as there have been in English) and this makes it difficult for the English speaker to trace earlier Irish words, including names. In 1956 there was a major spelling reform in Irish which simplified spelling, and among other things, dropped many silent letters. In modern Irish the word cluas is translated in English as "ear", and the word cluasaí is the equivalent of "listener" or "eavesdropper." The name Cluasach referred to some one with either acute hearing, or more likely, particularly large (or small) ears, and the name Lucey was derived from this.

In his letter, he noted that in The Journal of the Cork Historical and Archaeological Society (J.C.H.&A.S.) of 1920, a footnote (note 18, p. 74) says that the name Lucy has been said to mean "herb-doctor." 

Herb, s. Luibh f, lus m -a. Herbalist, s. 1. Luibheolai m. 2. Luibhlia m.

Ó Murchadha also mentioned that there was a minor Gaelic poet named Domhnall Ó Luasaidh, who probably lived in West Muskerry in the late 18th century; and he speculated that this spelling may be more correct than the "modern" ÓLuasaigh; and that if this is the case, the meaning would not be "herb-doctor." He went on to state that he does not know the derivation of that word, but suggested that it might be from luas, which means speed.

To further confuse the researcher: 

Cyril Ó Céirín, in Appendix II to his translation of Mo Scéal Féin, offers the following accurate but less than satisfying minor piece of information:

LUCY. Ó Luasaigh, a name peculiar to Cork until recently, whose origin is a puzzle.

Casey then mentions a second Anthony Lucy, son of a Thomas Lucy who settled (at an unspecified date) in Rafertan, Co Fermanagh. Next, he says that (another) Anthony Lucy settled at Magherstafanagh, Co Fermanagh, and was buried at Clogher Churchyard. From there he mentions an Anthony Dennis Lucey of Ballymodan Parish, in Bandon, Co. Cork who died in 1736, and who could be a son of the second (or perhaps he means third) Anthony Lucy.

Having made the aside that Cromwell’s supporters were in the Bandon area, Casey says that this to be the first mention of a Lucy in Counties Cork and Kerry that he has found. 

Our own research (albeit mainly into published documents) has unearthed mentions of an Olousy at Cashel between 1302 and 1307, and less contentiously, of Mac Clwos, Mac Cluosse and Ó Lwosie in Co. Cork of 1560, who predate the Cromwellians by perhaps a century.

Casey was trying to draw together the threads of what may have been, rather than simply listing what was known as fact. He freely conjectures (and makes it clear when he does so) and it would be easy to jump to the conclusion that somehow he’s simply being cautious in his choice of words. But when he says (p. 2407) The lineage, if true, could be as follows ..., we must accept his word that he’s guessing some parts.

Elsewhere, he provides a gold-mine of historical records, many of which are not available from other sources. It has been pointed out that while there are errors of transcription in his works, these are no greater than the errors that occur in many other transcriptions. They are utilised and quoted at length through out in this paper. 

May we take our own preference and consider the name to derive from a reference to ears, to a herbalist, or to speed, or must we accept that the origin has not yet been indisputably uncovered? I have come to the conclusion that the name has evolved from members of the Mac Carthy sept.

Between the years 1224 and 1510, a number of members of the Mac Carthy sept had the appellation Cluasach, or a variation thereof appended to their names. They included the following:

Sometimes, the cognomen Cluasach was used and the name Mac Carthy was omitted. This then, is the origin of the name, and the evidence of this usage and development is presented in the following part - 'Early references to the Name'.