The geometric, or Berry, phase effect is particularly diabolical when it is associated with a conical intersection of two states of the same symmetry. A recently developed algorithm for determining a conical intersection of two states of the same symmetry provides the basis for a general approach for characterizing paths that give rise to the geometric phase effect in this instance. This approach is used, with wave functions in excess of 1.5 million configuration state functions, to determine a conical intersection and associated paths in the Franck-Condon region of the photodissociation process CH3SH(X(1)A’)+hv-->CH3SH(1,2(1)A ")-->CH3S+H or CH3+SH. Derivative couplings along these paths are also determined. The implications for photodissociation dynamics are discussed.