The main subject of this research is the mechanism of semantic shifts in Latin borrowings in Early and Middle Irish. The appearance of Latin borrowings in Old Irish is traditionally associated with Christianization and the spread of monastic culture, but this is not entirely true. There is a relatively limited stratum of Latin lexemes that have been adopted by the Irish language through earlier contacts with Romanized Britain. They, as a rule, are distinguished by a special phonetics, demonstrating a more archaic cut of the receptor language. The work touches upon the problem of semantic changes in Latin borrowings within the framework of Irish culture and attempts to motivate them. The author proposes a scheme of "semantic shift" based on the original, latent, polysemy in the donor language. Earlier, the borrowing sén (<signum) developed in the Old Irish language an additional meaning - "sign, sign", and at the next stage of semantic derivation – "conspiracy, spell". Later, the doublet – sigin is used, as a rule, in combination sigin croiche ‘the sign of the cross’, and can be replaced in the same meaning by the original lexeme aired, which has a semantic field that almost coincides with latin signum.