Thomas Fitzwilliam, 1st Viscount Fitzwilliam,
He 
mar 23 Aug 1605 to   Margaret Plunkett
[descendant of  Edward III].
 
Knighted 23 Aug  1608.
[Ball, vol.2, 1903] tells of an incident in 1608
when the Lord Deputy's messenger delivered an order to the hall door of Merrion Castle,
"and how on returning to town he met Lord Fitzwilliam
at the cross roads at St. Stephen's Green
riding home with his wife and eight attendants".
 
Sheriff of Co.Dublin 1609.
 
He was  cr Baron Fitzwilliam of Thorncastle
and Viscount Fitzwilliam of Merrion
on 
 5 Aug  1629 by  Charles I.
Letters patent is in Wilton House.
 
Took his seat in the Irish House of Lords 
14 July 1634.
| 
 
Royalist in   Irish Confederate Wars 1641 to 1653:
 He was loyal to Charles I during English Civil War (the Irish Confederate Wars 1641 to 1653). Loyal to the crown in rebellion of 1641. Merrion Castle and Baggotrath were garrisoned with soldiers to defend Dublin. The government forces leader Simon Harcourt, who was mortally wounded by a sniper on 26 Mar 1642 while fighting the rebels at Carrickmines Castle, Co.Dublin, was taken to Merrion Castle where he died on 27 March 1642. Merrion Castle was overrun by the rebels in June 1642. He was cr Earl of Warrington by Charles I, 1 May 1645, but it was not upheld. This letters patent is in Wilton House. Merrion Castle was taken and was garrisoned by the Parliamentarians as at 1648. Baggotrath Castle was destroyed 1649 in the Royalist-Parliamentarian fighting leading to the Battle of Rathmines of 2 August 1649. 1st Duke of Ormonde and 1st Earl of Inchiquin were Royalist commanders. The Royalist defeat at Baggotrath and Rathmines was a crucial point in the end to resistance to Cromwell in Ireland. See [Ball, vol.2, 1903]. Cromwellian conquest of Ireland complete 1653. 1st Viscount was outlawed in Ireland under the Parliamentarians. He did not live to see the Restoration.  | 
 [Down Survey,  1655-58]
 shows      Lord Fitzwilliam  of Merrion, Irish Papist, 
 
 
as owner of 
Merrion
and Simmonscourt
and Owenstown.
This refers to him rather than his son
since his son is referred to as Col. Oliver Fitzwilliam.
  
He 
 died c.1655, age c.74 yrs. 
1st Viscount and Margaret 
had issue:

  
 
  
  Identified   in [Ball, vol.2, 1903]
 as Margaret Plunkett, 
 by Cornelius Johnson.  
  But  Fitzwilliam Museum now says "Portrait of a woman" by unknown. 
  Portrait c.1625. 
  Used here with the kind permission of the
Fitzwilliam Museum, Cambridge.
Dublin city officials ride in procession around the bounds of the city in  1603.
  They rode  to
Merrion chapel.
They   then  
came to
 the grounds of
 Merrion Castle.
They arrived at
"the south-west corner of the orchard ditch of Merrion,
through which corner the elder [fathers] of the city
said that of old time  they did ride.
And now  for  that   the same was so strongly fenced with trees and thorns,
which, in favour of the gentleman of the house of Merrion
 [Thomas Fitzwilliam],
being the city tenant,
they would loathly break down,
they rode a little besides it".
 
 
Gray's Inn in 1591. 
See  full size.
From here.
Gray's Inn in 1677. 
See  full size.
From here.

John Speed's map of   Dublin, 1610. View from S. 
From 
Wikimedia Commons.
See also
colour version
(printed 1896).
 
Copy of Speed's  1610 map that appears  as an insert  on
 John Rocque's map of Dublin, 1757.
This map is an insert  in Dublin Bay on Rocque's map.
Confusingly, he overlays  it with the depth figures he notes for the water,
which all have a dot after them.
See larger.
 

The Battle of Rathmines, 1649.
 
From 
 here.
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