Re your Allen ancestor who came to Prescott Co, Ont c1819. The Peter Robinson emigration group all came from southern Ireland, mostly county Wicklow. They settled near Lake Erie in Upper Canada, although a few remained in Lower Canada.
To have arrived in 1819, your ancestor was a very early arrival from Ulster indeed. Sounds to me like he could have been a military pensioner from the British Army, (they raised regiments in Ireland too), who after the full peace of 1815 were offered free land in Canada, in lieu of pension. Very few pensioners took up the offer. If that doesn't fit, then he may simply have been one of the earliest to get out of Ulster as the rural linen industry was collapsing, and sectarian violence began to escalate over Catholic Emancipation, for which debates began about 1819.
A few publications that might be of interest to you :
1. "Irish Migrants in the Canadas: A New Approach," by Bruce Elliot, McGill-Queens U Press, 1988. It deals with the organized migrations of 1815-1824 out of southern Ireland.
2. "Ulster Protestant Emigration to Lower Canada," by Gwen Barry (me). Evans Books, 2003. available from me at email@example.com
While it deals with Lower Canada, the conditions in Ulster mitigating for emigration, and the emigration patterns described from 1815-1835 apply equally to Upper Canada.
3. "Irish Emigration and Canadian Settlement: Patterns, Links and Letters," by Cecil J. Houston and William J. Smyth, U of T Press, 1990.
4. "British Emigration to British North America: the First Hundred Years," by Helen I. Cowan, U of T Press, 1961. It explains the British Army scheme from 1815-1830s to settle military pensioners, and the results of that scheme.
Most of the above should be availabe in Cdn university libraries.