We study a frustrated quantum Ising model relevant for Ca\(_3\)Co\(_2\)O\(_6\) that consists of a triangular lattice of weakly-coupled ferromagnetic (FM) chains. According to our quantum Monte Carlo simulations, the chains become FM and form a three-sublattice "up-up-down" structure for \(T \leq T_\text{CI}\). In contrast, long-period spin-density-wave (SDW) {\it microphases} are stabilized along the chains for \(T_\text{CI} < T < T_c\). Our mean field solutions reveal a quasi-continuous temperature dependence of the SDW wavelength, implying the existence of metastable states that explain the very slow dynamics observed in Ca\(_3\)Co\(_2\)O\(_6\). We also discuss implications of microphases for the related multiferroic compounds Ca\(_3\)CoMnO\(_6\) and Lu\(_2\)MnCoO\(_6\).