Inherited from Middle English -iere, probably from -ien (infinitival suffix) +‎ -ere (agent noun); i.e. nouns in -ere built on verbs in -ien; possibly reinforced by Old French -ier once -ie- became a rising diphthong. More at -ier, -er.

-yer

(rare, usually no longer productive) Agent suffix, equivalent to -er, appended to some nouns (especially those ending in the letter "w" or "v").
‎law + ‎-yer → ‎lawyer (AmE /ˈlɔɪər/)

‎saw + ‎-yer → ‎sawyer /ˈsɔːjər/