If it was a castle or an estate try www.landedestates.ie
Try Tullynally estate*.
There are lists of buildings if they have significant historical value. Can't think of any off hand and would need to google that.
You could try working for the early end and griffith valuation of the 1850's to see which of your relations lived where.
Do you have an OS map as I can't find either Clonave Island or Lough/Lake Deravaragh on my Road Atlas.
I could find the name Clonave in place names for Griffith Valuation is it is probably a Townland or a Union of. A second resource same. Found another resource and still nothing.
wiki-pedia Lough Derravaragh is located in Westmeath, north of Mullingar, between Castlepollard, Collinstown, Crookedwood, Multyfarnham and Knockeyon Hill.
An ancient St. Cauragh Chapel dedicated to St. Eyon, is surrounded by the communities of Clonva, Derrya, Kiltoom, Donore, Ballynakill, Streamstown & Knockabody.
On the Kiltoom side of the Lough, mounds of stones are part of artificial islands and gave protection to the people who lived in 'crannogs' on these Islands. In the 1970's a stoneage dugout was recovered from the lake by employees of *Tullynally estate.
Looking up some of the names above:
Derrya, Townland in the Parish of Mayne, Union of Granard
Donore, Townland in the Parish of Multynafarnham and
Ardnurcher AKA Horseleap Union of Mullingar
Ballynakill, townland in the Parish's of Kilcleagh Union
Athlone, Parish Kilkenny W. Union of Athlone, Parish
Multifarnham Union of Mullingar
Streamstown, Townland in the Parish of Ardnurcher Union Mullingar; Parish Faughalstown Union Castletowndelvin;
Parish Drumraney Union Ballymahon; Parish Noughaval Union
This is all I can do without people's names. A Castle in Ireland is not like a Castle in England and a large, large
house could be called a Castle, its just a name.
Tullynally Castle & Garden, previously Pakenham Hall, address is Tullynally Castlepollardwww.tullycastle.com
The Castle (it looks like one-certainly not a ruin) is 2 km from Castlepollard on Coole Village road, a British Army general Sir Edward Pakenham GCB. Has been the home of the Pakenham's 350 years.
Description fairy tale towers and turrets, gothic revival castle.
You Will find information on the tourist sites on Lough Derravaragh 14 km west of Mullingar with the village Multyfarnham on its southern shore and Castlepollard to the north east. 8 km in length. Good for fishing and best by boat.
Lakes/Loughs were used in the old days for transportation along with canals, like in England goods were transported along lakes and canals and that might explain the boathouse.
Have you looked in the 1911 Census for the address given there and occupation. www.census.nationalarchives.ie