Let $A,B,C$ be fixed points in the plane , and $D$ be a variable point on the circle $ABC$, distinct from $A,B,C$ . Let $I_{A},I_{B},I_{C},I_{D}$ be the Simson lines of $A,B,C,D$ with respect to triangles $BCD,ACD,ABD,ABC$ respectively. Find the locus of the intersection points of the four lines $I_{A},I_{B},I_{C},I_{D}$ when point $D$ varies.
1. **Understanding the Problem:**
   We are given a triangle \(ABC\) inscribed in a circle \((O)\). A point \(D\) lies on \((O)\) and is distinct from \(A\), \(B\), and \(C\). We need to find the locus of the intersection points of the Simson lines of \(A\), \(B\), \(C\), and \(D\) with respect to the triangles \(BCD\), \(ACD\), \(ABD\), and \(ABC\) respectively, as \(D\) varies on the circle.

2. **Simson Line Definition:**
   The Simson line of a point \(P\) with respect to a triangle \(XYZ\) is the line passing through the feet of the perpendiculars dropped from \(P\) to the sides of the triangle \(XYZ\).

3. **Lemma (Steiner's Line):**
   Steiner's line is a dilation of the Simson line with center \(P\) and ratio 2. The Steiner line passes through the orthocenter \(H\) of the triangle. Therefore, the Simson line passes through the midpoint \(K\) of \(PH\).

4. **Application of the Lemma:**
   - Let \(H\) be the orthocenter of \(\triangle ABC\).
   - Let \(H'\) be the orthocenter of \(\triangle MBC\), where \(M\) is a point on \((O)\).
   - Since \(AHH'M\) forms a parallelogram, \(K\) is the midpoint of \(AH\).

5. **Intersection of Simson Lines:**
   - The Simson line of \(M\) with respect to \(\triangle ABC\) passes through the midpoint \(K\) of \(MH\).
   - The Simson line of \(A\) with respect to \(\triangle MBC\) also passes through \(K\).

6. **Conclusion:**
   - The intersection point \(K\) of the Simson lines is the midpoint of \(MH\).
   - This midpoint \(K\) is the image of \(M\) under the homothety centered at \(H\) with ratio \(\frac{1}{2}\).
   - Therefore, the locus of \(K\) as \(M\) varies on \((O)\) is the nine-point circle (Euler circle) of \(\triangle ABC\).

\[
\boxed{\text{The locus of the intersection points of the Simson lines is the nine-point circle of } \triangle ABC.}
\]