"The farmer works on the farm."
Translation:Oibríonn an feirmeoir ar an bhfeirm.
feirm is eclipsed because it comes after the preposition ar and the singular definite article an, as explained in the Tips & Notes for the Eclipsis skill.
ar an bhfeirm, ag an bhfear, chuig an mbean, leis an mbuachaill, as an mbuidéal, roimh an gcluiche, etc. The gender of the noun is not relevant.