13 children generally requires a very early marriage age for the woman (16 or so) and often multiple births (twins often run in families) - what age was William's wife on marriage ? typical age for Manx marriages would be early 20's (he was 28) - you have 8 children in 20 years with no obvious gap for missing children and no history of twins (which is the other way of bumping up a family could) - 5 more would typically require at least 8 years - which for a woman in 40's is pushing fertility rather - gap between births generally increases significantly from late 30's.

Richard is not that common a Manx name - unusual names generalkly run in families (nor is Maria - rather too Catholic for Manx tastes at this period).
A second marriage on death of first wife might well account for a large family - but here there is often a significantr gap between children (few young umarried women unless they are desparate for support would take on a large widowed family.)
Arbory is in south of Island - few early Manx emigrants to Ohio came from south - most were from North of Island.