I had always connected "Athlone Town" with Co. Westmeath, but apparently it is shared with Co. Roscommon. There is a Civil Parish of St. Peter's, Co. Roscommon and the Civil Parish of St. Mary's, Co. Westmeath. There are Churches of Ireland with those same names.
What brought this to my attention was that I had spotted a Leonard family with all members "born in Co. Roscommon" living in 1901 in Athlone Town, Bastion St., St. Peter's Parish: Thomas Leonard (54) laborer, wife Mary (45), Patrick (21), John (19), and Kate (17).
Please see Ed Finn's Leitrim-Roscommon website for information on Leonards in 1901 and on Griffith's Valuation from 1857-58, etc. There are also references to Leonards other counties there.