A simple dual-core D-shaped plasmonic refractive index (RI) sensor with an open-arch channel is introduced in this paper. A thin plasmonic gold layer is inserted on the slotted portion, which makes the sensor cost effective. By introducing a ring in the flat surface of the D-shaped structure, the coupling effect is increased, which enhances sensor performance. The commonly used finite element method is applied to characterize sensor performance. Numerical investigation under the wavelength interrogation method shows maximum spectral sensitivities of 16,000 nm/RIU and 17,000 nm/RIU along with corresponding resolutions of 6.25×10-6RIU and 5.88×10-6RIU for x and y polarizations, respectively. In tandem with this, maximum amplitude sensitivities governed by the amplitude interrogation method are calculated at about 2,603.7000RIU-1 and 3,432.1929RIU-1 for x and y polarizations, respectively. The proposed sensor exhibits high figures of merit of 320RIU-1 and 283.33RIU-1 for x and y polarizations, respectively, in the RI detection range of 1.33 to 1.44. Moreover, the impact on sensitivity with the overall sensor behavior is analyzed by altering geometrical parameters such as pitch, air hole diameter, and gold layer thickness. So, with an eye toward sensor performance and economic viability, this sensor is assignable to bio-sensing applications.