The reduplicated s preterite is found in weak verbs of class A III. The endings are the same as those of the regular s preterite, except in the third person singular conjunct, which ends in -(a)e in the case of class A III a verbs (McCone's class H1) and in -(a)i in the case of class A III i verbs (McCone's class H2). The reduplicating syllable contains the vowel e, and as with the reduplicated suffixless preterite, the e can merge with a following lenited consonant to become é.
No deponent verbs are found in this class. As with other preterites, only active forms are built on this stem; passive forms are built from an old verbal adjective with a different stem.
See Category:Old Irish reduplicated s preterite verbs for a list of verbs belonging to this class.
A sample verb for the a verbs is baïd (“to die”). The endings are as follows:
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | |
|---|---|---|---|---|---|---|
| Absolute | bebsu | bebsai | bebais | bebsaimmi | not attested; probably bebsaithe | bebsait |
| Conjunct | ·bebus | ·bebais | ·bebae | ·bebsam | ·bebsaid | ·bebsat |
| Relative | bebas | bebsaimme | bebsaite |
A sample i verb ending in gníid (“to do, make”), where the preterite stem gén- is from *geɣn-, from *ge-gn-:
| 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. | |
|---|---|---|---|---|---|---|
| Absolute | génsu | génsai | génais | génsaimmi | not attested; probably génsaithe | génsait |
| Conjunct | ·génus | ·génais | ·génai | ·génsam | ·génsaid | ·génsat/·génset |
| Relative | génas | génsaimme | génsaite |