Good find with Patrick McNamara, I had searched unsuccessfully. Typically, the death records appear to be fairly accurate on the reporting of ages. But that might be because we only find the ones that are accurately reported. Perhaps when the informant is the youngest member of a large family, the information is less accurate? We saw this with the death record for Margaret Bowles McNamara who died in 1937 and according to her youngest son Ambrose, she was 74 years old. So Margaret was born about 1863? Margaret Bowles married James McNamara in 1870 and had her first child in 1871. Clearly the 1901 & 1911 census reports with her age reflecting a birth about 1850 are more accurate.
As far as finding the baptism record and determining the accurate age for Patrick McNamara, it is good to reflect on what we know for certain. In 1855 Griffith Valuation there was a man named Matthew McNamara in Uggoon who lived adjacent to McNamara lands in Kilmore. His property was passed to a man named Patrick McNamara who with Bridget Connors had a first born son named Matthew McNamara.
As you stated Patrick McNamara's death record is clearly incorrect, he did not die in 1941 at the age of 84 years. Using the age range from the 1901 census (age 53) and 1911 census (72 years) reports, Patrick could be born anytime between 1839 and 1848. Within this period are two missing pages of the Tulla baptism register covering May through July 1841, and January through March 1843.
It is possible that Matthew McNamara had a son Patrick in 1843. That would mean that Patrick McNamara died in 1941 at the age of 98 years. Very old, but not unbelievable. People from Tulla appear to live very long:
Also the Irish civil death records include the widow Ellen Vaughn of Callura, Tulla who died on 20 October 1906 at the age of 106 years:Mrs. Ellen MacIverne has died near Tulla, County Clare, at the age of 112 years.
Southern Cross, Adelaide, South Australia, 20 July 1906
"THE NEW ZEALAND TOUR." Southern Cross (Adelaide, SA : 1889 - 1954) 20 July 1906: 4. Web. <http://nla.gov.au/nla.news-article166967884>.
https://civilrecords.irishgenealogy.ie/ ... 555171.pdf
As far as John McNamara being the informant on the 1941 death record of Patrick McNamara, and being reported dead himself in American obituaries, I think we should revisit what was reported. When oldest brother Matthew died in 1935, his Washington DC obituary stated that he had two brothers living in Ireland but no names were reported. With just this information, you would assume the obituary was referring to the two brothers on the 1911 census: John and James. However, in 1953 when sister Anne Murphy died in Washington DC, the two living brothers were named specifically as Patrick and James on her obituary. It would be very easy for American relatives to confuse their uncles back in Ireland. I'm not sure who else the Patrick McNamara of Uggoon who died in 1941 could be, nor the informant John McNamara, and thus the American obituary from 1953 is most likely incorrect. Evidence to prove otherwise would be a death record for John McNamara between 1911 and 1935. It's interesting to note, that Patrick McNamara (born 1877) has not been found on the 1901 and 1911 Irish census. Most of the McNamara children arrived in America in the late 1890's and early 1900's. With their father living until 1941, you'd think a few of his children living in DC would have returned to Ireland at some point and a passenger listing might provide more information on Clare relatives.
I believe that Patrick McNamara is most likely the son of Matthew McNamara but require his 1868 death record as further evidence to support this theory. One thing to consider with attaching Patrick to a Martin McNamara family of Glendree is that we already have a Martin McNamara, a widower, who died in Uggoon in 1878; so I believe the two stories should coincide which might be a struggle. As far as finding the marriage of Patrick McNamara to Bridget Connors, it might be helpful to know where Bridget Connors is from; Winifred Connors who appears as a sponsor at the baptism of one of their children might provide an important clue.