On the 1901 Irish Census (per Ed Finn's Leitrim-Roscommon website) there is a Dowler household in Co. Roscommon but with a connection to Co. Leitrim. (I did not find a Dowler household in Leitrim on his 1901 database).

1901: Derreenargan Townland, Civil Parish Ardcarn, Barony Boyle, Co. Roscommon:

Sarah Dowler, age 50, (head) -- Born in Co. Leitrim (?maiden name). She states she is married (where is husband?) Also in home is Richard (26) a farmer, and Sarah (15).

I also note that a UK researcher (Curry) on a surname board at the L-R website is interested in the Dowler surname from that same area of Roscommon.