John Lowe died at 24 Marchioness Street, Belfast, of chronic bronchitis on 17 May 1926 at a reported age of 53, described as a married ex-soldier, the informant present at death was his daughter Mary of the same address, presumably the one who married Thomas McArdle 5 months later.
I couldn't find a death notice online for him (which might confirm burial details) but I wouldn't rule out Milltown Cemetery. BTW, I noticed that a son of James McCoo junior and Maria Hanlon was buried in public ground at Milltown in 1887, so others like James senior who died in 1900 might be in the cemetery too.
I have limited knowledge of what it's like dealing with Milltown Cemetery - hopefully others reading will have some knowledge and can advise you. According to the information in the following forum link, you can ring them up and they will answer simple queries for free, but anything more than that, like details of who else is in a grave with someone, will cost you.
https://www.belfastforum.co.uk/index.php?topic=83718.0