We present the results of a narrow-band (H₂S1,λ_c= 2.121μm,δλ= 0.021μm) imaging search with Wide Field Camera/United Kingdom Infrared Telescope for Hα emitters (HAEs) around several potential signposts of rare (~10^-7-10^-8comovingMpc^-3) overdense regions atz= 2.23: an overdensity of quasi-stellar objects [QSOs; 2dF QSO Redshift Survey (2QZ) cluster], a powerful, high-redshift radio galaxy (HzRG) and a concentration of submillimetre galaxies (SMGs) and optically faint radio galaxies (OFRGs). In total, we detect 137 narrow-band emitter candidates down to emission-line fluxes of 0.5-1 × 10^-16ergs^-1cm^-2, across a total area of 0.56deg² (2.1 × 10⁵comovingMpc³ atz= 2.23) in these fields. TheBzKcolours of the emitters suggest that at least 80per cent of our sample are likely to be HAEs atz= 2.23. This is one of the largest HAE samples known atz≳ 2. Although there is no evidence for large-scale (20comovingMpc) overdensities of the emitters around the targets, we find modest (~3σ) local overdensities associated with all three targets on smaller scales (5-10comovingMpc). In the 2QZ cluster field, the structure appears to be connecting the QSOs, while in the HzRG and SMG/OFRG fields, the structures are seen only in the vicinities of the targets. Our results suggest that these rare targets are located in local overdensities of galaxies, rather than average density regions, although it is not clear whether these structures are likely to evolve into rare, rich clusters. TheK-band magnitudes and the Hα equivalent widths of the emitters are weakly correlated with the overdensities of the emitters: emitters in overdense regions are more evolved systems compared to those in underdense regions atz= 2.23. We find several examples of extended HAEs near to the targets, including a striking example with a spatial extent of 7.5arcsec (60kpc atz= 2.23) in the 2QZ cluster field, suggesting that these are relatively common in overdense regions. We conclude that narrow-band Hα surveys are efficient routes to map overdense regions at high redshifts and thus to understand the relation between the growth of galaxies and their surrounding large-scale structures.