Is this another exception? It seemed like it should have been tosfaidh.
It's second conjugation. The imperative form is tosaigh.
DL says that the n in my answer containing tosnóidh was a typo. Was it?
I got caught again in adding an 'n'. Dineen has tosnuıġım which I recall from schooldays, [DL : tosaím]. I understand the spelling reform, but not the removal of the 'n'. I'd appreciate clarification.
Dialect. Tosnaigh is a Munster variant.