Hydrogen insertion in the CeNiSn Kondo semiconductor induces antiferromagnetic ordering for CeNiSnH below T_N = 4.5K whereas CeNiSnH_1.8 orders ferromagnetically below T_C = 7.0 K. We present a detailed investigation of these CeNiSnH_x compounds by means of muon spin spectroscopy performed at both ambient and high pressure (up to 23 kbar). It is shown that both magnetic ordering temperatures are sensitive to the applied pressure but in opposite directions, i.e., ^dT_C/_dp= -0.17 Kkbar^-1 for CeNiSnH_1.8 and ^dT_N/_dp = 0.016 Kkbar^-1 for CeNiSnH, respectively. This difference is discussed in terms of hydrogen-induced modification of the balance between the Ruderman-Kittel-Kasuya-Yosida (RKKY) and the Kondo interactions. The μSR study demonstrates that the CeNiSnH and CeNiSnH_1.8 are on different sides of Doniach's dome. The behavior of the CeNiSnH_1.8 is interpreted as resulting from an increase of the hybridization between 4f and 5d states and a concomitant steep increase of the Kondo temperature upon applying pressure. In the quest for a quantum critical point, an estimated pressure of about 35 kbar is expected to suppress the magnetism in CeNiSnH_1.8.