We study a 2D system of identical mobile particles on the surface of a cylinder of finite length d and circumference W , immersed in a medium of dielectric constant ε. The two end-circles of the cylinder are like-charged with the fixed uniform charge densities, the particles of opposite charge −e (e being the elementary charge) are coined as "counterions"; the system as a whole is electroneutral. Such a geometry is well defined also for finite numbers of counterions N . Our task is to derive an effective interaction between the end-circles mediated by the counterions in thermal equilibrium at the inverse temperature β. The exact solution of the system at the free-fermion coupling Γ ≡ βe 2 /ε = 2 is used to test the convergence of the pressure as the (even) number of particles increases from N = 2 to ∞. The pressure as a function of distance d is always positive (effective repulsion between the likecharged circles), decaying monotonously; the numerical results for N = 8 counterions are very close to those in the thermodynamic limit N → ∞. For the couplings Γ = 2γ with γ = 1, 2, . . ., there exists a mapping of the continuous two-dimensional (2D) Coulomb system with N particles onto the one-dimensional (1D) lattice model of N sites with interacting sets of anticommuting variables. This allows one to treat exactly the density profile, two-body density and the pressure for the couplings Γ = 4 and 6, up to N = 8 particles. Our main finding is that the pressure becomes negative at large enough distances d if and only if both like-charged walls carry a nonzero charge density. This indicates a like-attraction in the thermodynamic limit N → ∞ as well, starting from a relatively weak coupling constant Γ in between 2 and 4. As a by-product of the formalism, we derive specific sum rules which have direct impact on characteristics of the long-range decay of 2D two-body densities along the two walls.