Worth checking BDMUK (births deaths marriages). If you can find their marriage certificate (you'll need a volume number and page) you can order the certificate online for £7. This will tell you who her father was. It's worth bearing in mind that many Irish folk were coming over to work as labourers on the railways and canals during that period (due to the famines), however, a sizable number of McSweeneys were already here, having fought in the service of the British army (especially those from around Cork). Good luck!