If only dates can't be null you can put it like that:
IF ((p_END_DATE_1 >= p_START_DATE_2) AND
(p_START_DATE_1 <= p_END_DATE_2)) THEN
lv_RESULT := 'Y';
END IF;
when having nulls you can declare that, say
-Infinity == 1 Jan 1
+Infinity == 31 Dec 3999
So with a help of Coalesce
or Nvl
function you can implement
IF (coalesce(p_END_DATE_1, TO_DATE('31.12.3999', 'dd.mm.yyyy')) >=
coalesce(p_END_DATE_2, TO_DATE('31.12.3999', 'dd.mm.yyyy')) AND
coalesce(p_START_DATE_1, TO_DATE('1.1.1', 'dd.mm.yyyy')) <=
coalesce(p_START_DATE_2, TO_DATE('1.1.1', 'dd.mm.yyyy'))) THEN
lv_RESULT := 'Y';
END IF;