Kirsty, from the info you posted on censuses for the Galloways, Martha, mother, looks to have been born in Old Luce. This looks a possible for her christening - under McClieve:
MARTHA MCCLIEVE Christening on 13 FEB 1782 in Old Luce, Wigtown
Parents: DAVID MCCLIEVE and GRACE MCMILLAN
Apart from Martha, other children showing to parents David and Grace:
1. ANDREW MCCLIEVE Christening: 15 MAR 1780
2. PETER MCCLIEVE Christening: 12 FEB 1784
3. JEAN MCCLIEVE Christening: 22 AUG 1788
4. DAVID MCCLIEVE Christening: 26 NOV 1790
5. JAMES MCCLIEVEChristening: 04 MAR 1793 Old Luce
6. ANTHONY MCCLIEVE Christening: 07 MAY 1800
Submitted entry for parents' marriage on IGI:
GRACE MC MILLAN and DAVID MC CLIEVE Marriage on 27 JUN 1779 in Old Luce, Wigtown
Monica