The County Louth by-election of 1869 was contetsed on 11 January 1869. The by-election was held because the incumbent Liberal MP, Chichester Fortescue, became Chief Secretary for Ireland. Under the law at the time if an MP attained certain government positions, then they had to resign their seat and refight it. It was retained by Fortescue.
References
County Louth by-election, 1869 Wikipedia(Text) CC BY-SA