Hello Kat,
As far as I understood from the description, you have irregulary spaced Y ( assuming only Y is irregulary spaced and X is regularly spaced ) and then you have contourf plot on which you want see your irregularly spaced Y instead of default regularly spaced values.
This can be done in this way :
X = 1:5;
Y = [2,6,9,11,18];
Z = peaks(5);
ax =axes; % handle to the current axes
contourf(X,Y,Z);
ax.YTick = Y; %since Y is irregularly spaced and only that points must be seen on the axes