The Murchison Widefield Array (MWA) provides an excellent opportunity to extend the frequency range of pulsar observations in the Southern Hemisphere. In 2017, we carried out an initial census of a large sample of known (i.e., catalogued) pulsars at 185 MHz using the MWA Voltage Capture System (VCS). This led to the successful detection of 65 pulsars (including six millisecond pulsars and two in binary system), but using only ~10% of the full-array sensitivity, as the detected powers from the tiles were incoherently summed. In order to achieve the full-array sensitivity, a new tied-array (i.e. phased array) beam-former pipeline has been developed. Verification of the polarimetric performance of the MWA in the tied-array mode is required before we can use this new capability for pulsar science. Using two bright southern pulsars, PSR J0742-2822 (moderately polarised) and PSR J1752-2806 (highly polarised), we did a detailed characterisation of the polarimetric performance empirically. The result suggests that reliable pulsar polarimetry can be realised at frequencies < 270 MHz, and at zenith angles < 45 degrees. This analysis also led to first low-frequency polarimetric studies of these two southern pulsars, high-precision determinations of their Faraday rotation measurements, and an intriguing result, where the degree of linear polarisation of one of the pulsars, PSR J0742-2822, showing a steady (and rapid) decline at the low frequencies of the MWA, which is in contrast with the generally expected trend for pulsar emission. Motivated by this intriguing result, we undertook further analysis of de-polarisation in the low-frequency observations of multiple pulsars in the general direction of the Gum Nebula, including the well-known Vela pulsar, PSR J0835-4510, whose radiation possibly traverse through part of the nebular material. I will present the related analysis and new results and our current interpretation in the context of magneto-ionic turbulent ISM in the general direction of the nebula.