Old Irish is

← Proto-Celtic *esti (inherited)

← Proto-Indo-European *h₁es- (root)

← Proto-Indo-European *h₁ésti (inherited)

← Proto-Indo-European *h₁es- (mention)

← Proto-Indo-European *bʰuH- (mention)

→ Welsh ys (cognate)

→ Irish is (inherited)

→ Scottish Gaelic is (inherited)

→ Manx s' (cognate)

→ Old Irish fa (mention)

→ Old Irish combad (mention)

→ Old Irish indat (mention)

→ Old Irish indat (mention)

→ Old Irish combin (mention)