Complex sentence: Gore-Booth had been a Sligo MP since 1850, and his return was considered almost certain; effectively, this was a contest between Cooper and O'Connor, and it was him who the Liberals sought to unseat.
Split into simpler sentences: Gore-Booth had been a Sligo MP since 1850, and his return was considered almost certain. This was effectively a two-way contest between Cooper and O'Connor, and it was him who the Liberals sought to unseat.