The language that came before Ancient Irish Gaelic can be called "Celtic".

Celtic, and the later Gaelic languages, have always been in the 
Indo-European family of languages.

The short answer is: Ogham was probably used to write Gaelic, 
and later "Irish"
(from Old Irish Gaelic to modern Irish Gaelic).

Some background:

circa 12000 B.C.:
The Common Celtic language was probably very similar to its 
relatives Italic, Illyrian and Venetic. Celtic words and 
word elements were borrowed by Slavic and Germanic languages 
in this very early epoch. 

circa 600 BC:
Goidelic-speaking Celts from Spain arrive in Ireland.

[The most ancient remnants of a celtic dialect in written 
form have been found in northern Italy 
(Sesto Calende, ~600 b.C., Castelletto Ticino, ~575-550 b.C.). 
It is a relatively recent acquisition that these (Lepontic)
 inscriptions are actually written in a celtic dialect 
(Lejeune, "Lepontica", 1971).]

4th century B.C.:
Invasion of "Q-Celtic" speaking peoples to Ireland 
(probably from Western France)

("P-Celtic" grew out of invasions from Ireland to southern 
England and Wales, and from there (in 5th century A.D.) to Brittany.)

Note that the language evolved though: the "Gaels" (after 
whom Gaelic is named)
did not arrive in Ireland until about 100 B.C. 
(some scholars believe as early as 300 B.C.).
In other words, the form of Celtic that was to become 
Irish was brought to Ireland by the invading Gaels.


You can find more info at
http://www.celticcrossroads.com/index1.htm
http://www.siliconglen.com/celtfaq/1_3.html

           Pádraig O'flaithbheartach
             ( Patrick Flaherty )