Greetings from the Academy of Saint Gabriel!
You asked for our opinion of the names <Tristam> and <McFarland>, which you want to use in a 12th century Irish name. Here is what we have found.
Unfortunately, <Tristam> is not an Irish name. In fact, this particular spelling does not appear to be correct. The normal English forms of the name are <Tristram> and <Tristan>; <Tristan> is also found in French from the 12th century onward . <Tristam> appears to be an incorrect combination of the two historical forms.
The name is recorded in England and Scots-speaking regions of Scotland in various form from the late 12th century. (Scots was a language closely related to English, distinct from Scottish Gaelic.)
Tristram filius Trestram (1200-1220, "Tristram son of Trestram")  Tristram Cementarius (1204) 
The Gaelic-speaking Irish were familiar with Arthurian literature, including the story of Tristan and Iseult, but their versions of that story do not use the name <Tristan>.
The ultimate source of these names is not entirely clear; our sources believe that it is a Brythonic Celtic name <Drystan> [1, 2, 4]. The derivation is obscured by the transmission of the name through Old French, where it was confused with the word <tristis> "sad" . There is evidence of <Drostán> and <Drustagnos> in early Cornish and Scottish Gaelic inscriptions, but we find nothing later than the 7th or 8th century and nothing in Irish [1, 6].
<McFarland> is a modern anglicization of the Gaelic patonymic <mac Pharthaláin>, a name found in Ireland chiefly in Tyrone, Armagh, and Leitrim, and also in Scotland . It is means "son of Parthalán"; <Parthalán> is often associated with the Biblical <Bartholomaeus>, but may be an unrelated native Gaelic name [8, 9, 1]. In Scotland, it is recorded in 1385 in the name of <Malcolm Mcpharlane>
If you want a 12th century Gaelic name, then <Tristram> is probably not an appropriate choice for a given name. The name was unknown in Gaelic and would not have been combined with a Gaelic patronymic like <mac Pharthaláin> at that early date. The name might well have been used among the Anglo-Normans who settled parts of Ireland, but they did not use native Gaelic patronymics. If you would like to consider other Gaelic given names which match your Gaelic patronymic, we would be happy to suggest some possibilities.
On the other hand, if you want to use <Tristram> or <Tristan> in a historically-correct name, then you'll need to change your persona might have been used by English settlers in Ireland. If you choose this approach, then <mac Pharthaláin> would not be an appropriate byname; instead you should choose an English byname. The most common types of byname in 12th century English were based on one's father's name (e.g. <Tristram William> "Tristram son of William"), one's place of residence or birth (e.g. <Tristan de Perton>), one's occupation (<Tristram le
Sergeant>), or some personal description (<Trestram Blanke>, "Trestram
White"). If you have a specific meaning you'd like to convey in your byname, give us the details and we'll try to provide you a form appropriate to your period.
I hope this letter has been useful. Please write us again if any part of pit has been unclear or if you have other questions. I was assisted in researching and writing this letter by Talan Gwynek and Charles O'Connor.
For the Academy,
 Black, George F., _The Surnames of Scotland: Their Origin, Meaning and
History_, (New York: The New York Public Library, 1986).
 Withycombe, E.G., _The Oxford Dictionary of English Christian Names_,
3rd ed. (Oxford: Oxford University Press, 1988).
 Bardsley, Charles, A Dictionary of English and Welsh Surnames
(Baltimore: Genealogical Publishing Company, 1980).
 Reaney, P. H., & R. M. Wilson, _A Dictionary of English Surnames_
(London: Routledge, 1991; Oxford University Press, 1995).
 Hanks, Patrick & Flavia Hodges, _A Dictionary of First Names_ (New
York: Oxford University Press, 1990).
 Watson, William J., _The History of the Celtic Place-Names of
Scotland_ (London: William Blackwood & Sons Ltd., 1926).
 Woulfe, Patrick, Sloinnte Gaedheal Is Gall: Irish Names and Surnames
(Kansas City: Irish Genealogical Foundation).
 O/ Corráin, Donnchadh and Fidelma Maguire, _Irish Names_ (Dublin: The
Lilliput Press, 1990).
 MacLysaght, Edward, _More Irish Families_ (Blackrock, Co. Dublin: Irish
Academic Press, 1982).