there were several Simpson families in or around Douglas - one that might be worthy of further investigation given the 'Irish guards' and maritime connection is the following large family from 1881 census
Onchan SIMPSON Samuel 8 Heywood Place Head M 41 Steam Packet Porter born Portaferry Ireland
Onchan SIMPSON Sarah 8 Heywood Place Wife M 41 General Dealer Douglas IoM
Onchan SIMPSON Henry 8 Heywood Place Son U 18 Steam Packet Porter Douglas IoM
Onchan SIMPSON Samuel 8 Heywood Place Son U 15 --- Douglas IoM
Onchan SIMPSON Joseph 8 Heywood Place Son U 13 Scholar Douglas IoM
Onchan SIMPSON David 8 Heywood Place Son U 11 Scholar Douglas IoM
Onchan SIMPSON Andrew 8 Heywood Place Son U 8 Scholar Douglas IoM
Onchan SIMPSON Edward 8 Heywood Place Son U 2 --- Douglas IoM
Onchan SIMPSON Sarah 8 Heywood Place Daur - 14 --- Douglas IoM
Onchan ESON Richard 8 Heywood Place Relative U 13 --- Douglas IoM
Onchan COLQUIT Mary 8 Heywood Place Servant W 46 Domestic Servant Peel IoM

however since you now have birth year then apply to General Registry for a birth cert - will give mother's name etc , then go for marriage certificate + with a bit of luck you may find them in 1901 census.