We are using μSR to probe the evolution of magnetism from single-ion to spin glass behaviour in LiY_1-xHo_xF₄ with increasing x. For x = 0.002 the Ho spin dynamics exhibit single-ion behaviour similar to that observed in molecular magnets. Near T ≈ 10 K, the energy gap between the Ho ground state electronic doublet and first excited singlet energy levels, we find that the longitudinal depolarization rate exhibits a peak that depends on the Larmor frequency. At temperatures T << 10 K we find sharp reductions in the transverse field depolarization rates in the vicinity of magnetic field induced (avoided) level crossings (ALCs) in the hyperfinesplit ground state energy manifold. This is attributed to a reduction in the quasistatic magnetic disorder due to resonant Ho spin fluctuations at frequencies much larger than the muon Larmor frequency. As the Ho concentration is increased toward x = 0.05, magnetic correlations are observed to become significant, changing the 10 K peak into a plateau, and significantly broadening the rate changes observed at ALCs.