Hi Elizabeth
12 SEP 1840: No 23: Marriage solemnized at the Parish Church in the Parish of Lonan in the Isle of Man: ROBERT CASEMENT, 25, in tolerable circumstances#, a Millwright of Lonan, father John Casement, Millwright AND AGNES THOMPSON, 23, of Lonan, father William Thompson, Paper maker. // Married by : - Joseph Qualtrough Vicar of Lonan // Witnesses: - John Kinley; TW Kerruish.
# note JQ's humour - instead of "Bachelor" he gave an assessment of his affluence!

Mary Thompson was quite likely the sister of Agnes - especially with "Casement" included in one of their son's names; her husband John Milburn died in 1860 aged 40yrs and she remarried the widower Robert Radshaw at Braddan 1862-12-17. Robert died at Dumbell's Row Laxey in 1877 aged 45yrs [bd Dec 10], and Mary Readshaw of New Road Laxey was buried 1894-01-27 Lonan aged 72 yrs.
Census Returns:
1851: HO_1007_2524_457_122: Laxey Village [?Shore Hotel?]: John Milburn, Hd, Md, 30, Inn Keeper, b.Lonan; Mary Milburn, Wife, Md, 28, b.Scotland; William Milburn, Uncle, Un, 51, Pilot for Laxey, b.Lonan; Deborah Milburn, Barmaid, Un, 22, Barmaid, b.Lonan; Margaret McVoy, Servt, Un, 18, Housemaid, b.Ireland.

1861: RG_09_4423_70_112; Laxey: Mary Milburn, Hd, Widow, 35, Fund Holder, b.Scotland; John H Milburn, Son, 10, Scholar; George B Milburn, Son, 7; Mary A Milburn, Dau, 6; Jane Milburn, Dau, 4; Isaac Milburn, Son, 2; Jane Quayle, Servt, Un, 15, Nurse; all b.Lonan.

1871: RG_10_5774_63_136; 14, Dumbell's Row [Laxey]: Robert Readshaw, Hd, Md, 41, Lead Miner, b.IoM; Mary Readshaw, Wife, Md, 45, b.Scotland; George Milburn, Wife's Son, S, 18, Labourer; Isaac Milburn, Wife's Son, 12, Scholar; Annie Milburn, Wife's Dau, S, 16, Genl Servt; Jessie Readshaw, Dau, 7, Scholar; all b.IoM.

1881: RG_11_5601_60_p5; New Road [Laxey]: Mary Redshaw, Hd, Widow, 55, Provision Dealer, b.Scotland; Isaac Milburn, Son, S, 22, Formerly Lead Washer, b.IoM.

Now, unfortunately, comes the crunch!!:
1891: RG_12_4686_61_92; New Road [Laxey]: Mary Readshaw, Hd, ?Wife?, 68, Living on her own means, b.Scotland FIFESHIRE; Mary Ann Milburn, Dau, S, 35, Dressmaker, b.IoM; Isaac S Milburn, Son, S, 31, Living on his own means, b.IoM; Annie James, Lodger, S, 29, Milliner, b.England Manchester.

Robert Casement is one of my 1st Cousins, 4x removed; I had tried to pursue Agnes' parents, and reached the same tentative conclusion as David, but now it seems that perhaps there is a Fifeshire connection!

JohnL