Given the premise that Thomas Looney is the father of both illegitimate Lonan children of Elinor Costean, then a Thomas b 1773 is too young. He'd be only 13 in 1786 and 18 in 1791.

I think there are 2 candidates, one much better than the other. Eliminating the weak one first, it would be the Thomas bapt 17 Aug 1748 Maughold s/o John L & Margaret Kelvie. There is a baptism for him in both Maughold and Ramsey, same day. He may be the best candidate to be the father of Margaret bp 12 Dec 1773 Maughold d/o Thomas Looney & Jane Kermeen (illegitimate?). Two years later he married Isabella Moore and 8 children followed. Very unlikely he wandered over to Lonan.

I prefer Thomas Looney bp 8 Jul 1740 Lonan, s/o Daniel Looney & Jane Kissack of Ballamoar. He was buried 22 Apr 1819 Lonan age "80". I haven't found a marriage for him, but a Thomas Looney had children in Lonan: Ann 1769, Jane 1770, Thomas 1773, James 1777. The image for 1770 Jane had "Ball.." [for Ballamoar?] after Thomas Looney's name before a tear eliminated the rest of it. The MNB extract for Thomas's father Daniel 1767 says that Thomas the eldest has a settlement.

Just spotted on the IGI an "Intended Marriage" 5 Aug 1767 at Liverpool for Thomas Looney of Liverpool batchelor age 30 & Jane Ward of Liverpool spinster age 20.

A will extract of a different Thomas Looney 1789 Lonan had "friend Thomas Looney of Ballamoar exor" in the MNB. Jane Looney als Cowin had similar in 1792. They married 1750/51 Lonan, and I think they were uncle and aunt of Thomas b 1740, likely childless.

With regard to Elinor Costean, a possible burial on Family Search: 1834 Maughold "Elinor Costain Ramsey 77 March 25". The place Ramsey puts her in the same place as John and Ishmael Looney, and the age works back to about 1756. A possible baptism in 1756 Lonan: "a Child of Danl Costean baptized Jan 16". Daniel Costean married Margaret Fayle 2 Jun 1728 Andreas, and then Margaret Garrett 3 Apr 1844 Onchan. There's well over a dozen children from 1729 to 1765 across the 2 marriages. Daniel Costean was buried 13 July 1767 leaving a widow with at least 11 children aged 2 to 22, not counting a couple of older step children.

My theory is that something happened to Thomas Looney's wife after the last child 1777, and he took up with Elinor Costean but never married her. I haven't spent any time researching his legitimate children, but I believe Thomas Looney b 1773 would be his heir and the family continued at Ballamoar for a long time after.