Do you have any other information? Years? Deaths? Place of marriage to Mullins? Names of any children?
I found this marriage on rootsireland.ie, but without more info I can't say that it is your ancestors. Note that this Patrick was slightly younger than yours, though ages in records like this are not reliable.
You mentioned Shannon. This Patrick's home place of Sheanagh sounds similar.
Marriage: 16-Jan-1864
Parish / District: BALLYLEAGUE, Co. Roscommon
Patrick Shea of Sheanagh, farmer, 27, first marriage
Margaret Kelly of Cloonlar, 28, first marriage
Husband's Father: Richard Shea, farmer
Wife's Father : James Kelly, farmer
Witnesses: Patrick Hyland , Catherine Hedian
Fr Thomas Philips Pp: Roscommon Church: Both Catholic.