The s preterite is the most common preterite class in Old Irish. All weak verbs of classes A I and AII as well as a few strong verbs take the s preterite. It goes back to the Proto-Indo-European sigmatic aorist found in Sanskrit (e.g. तुदति (tudáti, “(s)he strikes”) → अतौत्सीत् (átautsīt, “(s)he struck”)), Ancient Greek (e.g. λῡ́ω (lū́ō, “I loosen”) → ἔλῡσα (élūsa, “I loosened”)), Latin (e.g. carpō (“I seize”) → carpsī (“I seized”)), Old Church Slavonic (e.g. хвалите (xvalite, “you pl praise”) → хвалисте (xvaliste, “you pl praised”)), Welsh (e.g. caru (“to love”) → cerais (“I loved”), caraswn (“I had loved”)) and elsewhere.
In Proto-Celtic, the suffix must have contained geminate ss, since singleton s between two vowels became h in Brythonic and disappeared in Goidelic. This ss originated in the third-person singular, where the sequence *-s-t became *-ss by regular sound change; this ss then spread by paradigm leveling to the other forms.
The passive forms are not built with the s suffix; rather, they are derived from a verbal adjective with a suffix starting with t.
See Category:Old Irish s preterite verbs for a list of verbs belonging to this class.
A sample verb for this class is gaibid (“to take”), which ends in a broad consonant. The endings are as follows:
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Absolute | gabsu | gabsai | gabais | gabsaimmi | not attested; probably gabsaithe | gabsait | gabthae | gabthai |
| Conjunct | ·gabus | ·gabais | ·gab | ·gabsam | ·gabsaid | ·gabsat | ·gabad | ·gabtha |
| Relative | gabas | gabsaimme | gabsaite | gabthae | gabthai |
A sample verb ending in a slender consonant is léicid (“to leave”):
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Absolute | léicsiu | léicsi | léicis | léicsimmi | not attested; probably léicsithe | léicsit | léicthe | léicthi |
| Conjunct | ·léicius | ·léicis | ·léic | ·léicsem | ·léicsid | ·léicset | ·léiced | ·léicthea |
| Relative | léices | léicsimme | léicsite | léicthe | léicthi |
A sample verb for this class is molaidir (“to praise”). The conjunct endings are as follows (no absolute endings are attested for deponent verbs with a root ending in a broad consonant):
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Conjunct | ·molsur | ·molsar | ·molastar | ·molsammar | ·molsaid | ·molsatar | ·molad | ·moltha |
If the root ends in a consonant cluster, an epenthetic vowel is inserted between the cluster and a consonant-initial ending, as in labraithir (“to speak”):
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Conjunct | ·labrasur | ·labraiser | ·labrastar | ·labrasammar | ·labraisid | ·labrasatar | ·labrad | ·labratha |
Verbs ending in a slender consonant (including all those using the suffix -igidir) are conjugated like foilsigidir:
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | Passive singular | Passive plural | |
|---|---|---|---|---|---|---|---|---|
| Absolute | — | — | foilsigistir* | — | — | — | — | — |
| Conjunct | ·foilsigsiur | ·foilsigser | ·foilsigestar | ·foilsigsemmar | ·foilsigsid | ·foilsigsetar | ·foilsiged | ·foilsigthea |
| *Very rarely attested; examples include cíchnaigistir (“made a loud, unpleasant noise”) and búiristir (“roared”). | ||||||||