Old and Middle Irish have a wide array of terminology to signify what we'd call a "ceasefire":

• Essomon: lit. 'a cessation of fear', used in a martial context.
• Ossad: a respite from fighting.
• Aslach: one possible definition depending on context is "petitioning for a parlay" or more simply, a "truce".
• Cairde: usually a legal term referring to a pact or treaty between two or more entities within a given territory, the opposite of a Cain or all-encompassing law. Depending on the context, this term can also signify a truce or respite from fighting. The term survives into Modern Irish as the word for "friends" or "friendship".
• Síd, síth: a general term for a period of peace, anywhere between a temporary truce to a longlasting peacetime. This term is just a general analogue for "peace" and can be used in a wide variety of contexts.

If you'd like to find some similar terms for yourself, have some fun with the electronic Dictionary of the Irish Language (eDIL) which contains a massive variety of terms including the sources they appear in and in which contexts.