The Contemplator site http://www.contemplator.com/carolan/carlnbio.html#name gives this explanation:



Carolan vs. O'Carolan

I am often confronted with the opinion that my site should refer to Carolan as O'Carolan. I originally used "Turlough Carolan" because Grainne Yeats - certainly an expert on the man - refers to him as such. However, as this site relies strongly on Donal O'Sullivan's biography of Carolan I have decided to use his naming conventions for Carolan, except where I am quoting another source. There is a great deal of confusion and inconsistency in naming Carolan and so I have decided to devote some space to O'Sullivan's explanation.

Carolan's full name in Irish is Toirdhealbhach Ó Cearbhalláin. As is often the case with names of the past, several variations of both the first and last name appear. In English the name would be Turlough or Terence Carolan.

According to O'Sullivan, when full names (first and last together) are written in Gaelic it is customary to add the Ó prefix. However, in using the surname alone, O'Sullivan states, one should use the form the owner and his friends used. In his songs for Fallon and John Stafford, Carolan referred to himself as Cearbhallán, not Ó Cearbhallán. In his elegy for Carolan MacCabe uses the same, as do several other close friends in writing of Carolan. Writing in English they refer to him as Carolan - not O'Carolan. O'Sullivan, therefore, feels certain that Carolan was known to himself and his friends as Cearbhallán or Carolan.

So, without meaning to offend anyone Gaelic or the bard himself, I will use the O'Sullivan conventions. When using the full name I will use Turlough O'Carolan and when using the last name alone I will use Carolan.

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So ye see, he's really Terry Carolan!!