noun
Refers to the sun as it sets in the evening, often with a poetic or literary feel.
入り日が山の向こうに沈んでいく。
The setting sun sinks behind the mountains.
夕日 is the common word for 'evening sun' or 'sunset'. 入り日 is more literary and less frequent in everyday speech.
Compound of 入り (iri, 'entering') + 日 (hi, 'sun'), literally 'entering sun', referring to the sun going down.