Ogham was used to write the very first Irish languages. It was a medieval alphabet. Inscriptions that were found were mainly personal names, but also some words. (here provided with meanings and translations to English)

Irish Ogham Celtic & Indo-European
anm (a name) IE *nem- (to call, to assign), *@nomon- (a name), Common Celtic (CC) *anmen
avi (a grandson, a grandfather) CC *avios, Latin avus (a grandfather), Old Irish ua
brana (a raven) IE *worn- (a raven), Slavic *vorna (a crow), Tocharic wraun’a (a crow), Lithuanian varna
celi (a servant) CC *keiljo- (a way-farer)
coi (here)
cuna (a wolf, a dog) IE *kwon- (a dog)
eqa (a horse) IE *ekwo- (a horse)
essi (he is) IE *es- (to be)
iarn (iron) IE *ayos-, *ayes- (metal), CC *eisarno- (iron), Gaulish isarno- (iron)
inigena (a daughter) IE *en (in, inside), *g’en- (kinship), CC *enigena
iva (a yew) CC *ivos (a yew), Lithuanian ieva (a bird cherry tree), Russian iva (a willow), Old High German iwa (a yew), Armenian aigi (a vineyard)
lie (a stone) CC *levink- (a stone), Old Irish lia, German lei (a stone, a rock), Italian lavagna (a slate)
maguno (a servant, a slave) IE *maghu- (a young person), CC *makw-, Gaulish magus (a servant)
maqi (a son) IE *maghu- (a young person), CC *makw-
me (my) IE *mé (me, mine)
mosac (a boy)
mucai (a descendant)
tovisaci (a leader) IE *wedh- (to lead), CC *to-vessiko- (a leader), Irish Taoiseach (Prime Minister)
tria (three) IE *treyes, *tri (three)
vail (a wolf) CC *vailo- (a wolf), Armenian gail (a wolf)
vics (a warrior) CC *vikt-, Old Irish fichim (I fight), Latin vinco (I win), Gothic veihan
velitas (a poet) IE *wel- (to see), CC *velit- (a one who sees), Germanic Veleda (a prophetess)