I have a short page about Lacy’s Canal, which runs from the south of the town of Mullingar to Lough Ennell (or vice versa).
Some folk say that the canal was named after Hugh de Lacy, a twelfth-century Lord of Meath, even though it was built in the eighteenth century. I have to say that that sounds improbable to me: I see no reason why the builders (or excavators) should thus summon the ghost of a long-dead lord, and I know of no evidence for the assertion. I accept, of course, that it may exist and, if so, I would be glad to hear about it.
However, it seems to me to be more likely that the canal was named after the eighteenth-century person who built it, who owned the land or who ran a business selling turf. I was therefore interested to read this advertisement in Saunders’s News-Letter of 29 April 1829:
COUNTY WESTMEATH
To be sold, the interest in a lease for three young lives or 26 years unexpired, of about 50 acres of the lands of Grange, adjoining the Royal Canal, and the Great Barrack now building, and within half a mile of Mullingar: the lands are of prime quality, and one of the best situations in Westmeath for a lodge, dwelling, farm house, or dairy, there being the materials of a mansion upon the premises which would build it upon a site commanding an extensive prospect of the beautiful lake and improvements of Belvedere and Rochfort.
The crops of oats and potatoes can be had at a valuation. A purchaser of this interest will acquire many other advantages; immediate possession can be given when the value is offered. Apply to Edward Lacy, Mullingar, or Mr Charles Crampton, No 45, Clarendon-street, Dublin.
Grange and Mullingar (OSI ~1840)
Thus it seems that there were folk (or was at least one person) called Lacy living near Mullingar in the early nineteenth century. Unfortunately the Landed Estates Database does not cover Leinster, so I have no more information about them (or him).
Irish waterways history 
The name “Grange” was also applied to a rather nasty council housing estate in Mullingar (in which I inadvertently purchased my first home) even though, looking at the OSI maps online and overlaying the modern map with the 6″, is actually within the boundary of “Mullingar” and not in either “Grange South” or “Grange North”.
However, based on the text above, if the lands in question adjoined “the Great Barrack”, this would include the land on which the house I bought was located (hence giving me ownership of part of it). I sold out after 5 years so no longer have an economic connection to Mullingar.